NAIROBI, Kenya — Kenya's military caused the collapse of three floors of the Westgate Mall in the deadly terrorist siege, a top-ranking official disclosed Friday, while the government urged patience with the pace of an investigation that has left key questions unanswered.

Seven days after 67 people were killed in the attack on the upscale shopping center, there is still no clear word on the fate of dozens who have been reported missing and no details on the terrorists who carried it out.

The account of the roof collapse raises the possibility that the military may have caused the death of hostages in its rescue attempt. An undisclosed number of people are feared to be buried in the rubble.

The official said autopsies will be conducted on any bodies found to determine the cause of death – from the militants or the structural collapse. The high-ranking government official spoke to The Associated Press on condition of anonymity because he was not authorized to divulge sensitive information.

The official also confirmed that Kenyan troops fired rocket-propelled grenades inside the mall, but would not say what caused the floors to collapse, if the action was intentional, or if it was an accident.

The account at least partially backs up information given to AP on Wednesday by another official who said RPGs fired by soldiers created a gaping hole in the mall's roof and caused the floors to collapse.

Four huge explosions had rocked the mall Monday and dark smoke poured out – the likely time that the floors collapsed.

A soldier who was returning from the mall Tuesday while carrying a rocket launcher told the AP reporter that he had fired it inside. The soldier spoke on condition of anonymity because he was ordered not to talk to the media.

The government has not said publicly what caused the collapse. One official had earlier suggested it was caused by a mattress fire in the Nakumatt department store.

Presidential spokesman Manoah Esipisu said structural engineers are examining the collapse. FBI agents, along with investigators from Britain, Canada and Germany, are participating in the inquiry. Results are not expected until next week at the earliest.

Police are trying to determine if the attackers stored ammunition in the mall hours or even days before the attack, and investigators are tracing the ownership of a car that has been discovered and is believed to have been used by the gunmen.

Al-Shabab said it carried out Saturday's attack to punish Kenya for sending its troops into Somalia to fight the al-Qaida-linked militant group that had seized large parts of that country for years before being dislodged from the capital, Mogadishu.

U.S. Ambassador Robert F. Godec said the United States is concerned about the specter of more violence from al-Shabab.

"Obviously they do pose a threat, and it's critically important, I think, that we understand al-Shabab, understand what the terrorists in that organization are up to, how they carry out attacks, and really seek to frankly end the threat that the organization poses," Godec said in an interview with AP. "So we are working very hard with Kenya, and other countries, to do so."

Amid the possibility that some of the attackers may have escaped during the evacuation of civilians from the mall, authorities have increased surveillance at border crossings and at the Nairobi airport, the senior government official said.

Eight suspects are being held over the attack, Interior Minister Joseph Ole Lenku said. Three others who had been detained were released.

The government says at least 61 civilians and six security forces were killed. At least five attackers also were killed.

At the request of Kenya, Interpol on Thursday issued a notice asking for help in capturing 29-year-old British-born fugitive Samantha Lewthwaite – not in connection with the mall attack, but over a 2011 plot to bomb holiday resorts in Kenya.

Known in British tabloids as "the white widow," she was married to one of the suicide bombers in the 2005 attack on London's transit system, and British media have speculated she was involved in the mall attack.

But Lenku suggested that Interpol raised the issue of the international alert. He declined to discuss any links to the mall attack, saying he "wouldn't want to pre-empt the position of the forensic investigation taking place."

Dramatic new video filmed by the Kenyan Red Cross shows responders rushing to help the injured shortly after the assault began on Sept. 21.

The footage, obtained by The Associated Press shows responders entering the mall's upper level parking area and treating injured amid bodies on the pavement.

The end of the graphic video appears to show several seriously wounded and dead people where a children's cooking event had been taking place. Injured people are wailing and calling out for help.

Inside the mall Friday – the last of three days of an official mourning period – a baby stroller could be seen overturned on the marble floor next to wilting fresh flowers at a florist's shop. Slabs of concrete sat on top of flattened cars in the parking area. Elsewhere, there were rows of scorched vehicles.

Kenyan officials have offered at times contradictory accounts of the siege and are reluctant to release many details of the investigation prematurely.

Lenku urged patience, saying the investigation is "a very delicate and complex operation that requires time."

He said no bodies have been recovered from the rubble and no official reports of missing persons have been filed. The Kenyan Red Cross has said 59 people are unaccounted for, raising fears of bodies in the debris.

The possibility that government troops were behind the roof collapse only added to the frustrations of Kenyans questioning what happened at the mall.

"You have to blame the government, not the soldiers, because they had to get their orders from somewhere," said John Odera, a security guard. "They should be held responsible for what they did."

A user-generated list of more than 80 questions on social media includes "How many terrorists were involved in the attack?" "Are any terrorists loose in the city?" "Can we see the bodies of the `neutralized' terrorists?" and "Are we still safe?"

The list reflects a broader frustration in Kenya over the lack of information about the attack.

Boniface Mwangi, a photographer who has emerged as one of Kenya's leading political activists, said he believes the lack of a clear accounting is due to the government trying to paper over shortcomings in its handling of the operation.

"They're trying to cover up something," he said. "If it's true you have nothing to hide, let's know what really happened."

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Associated Press reporters Ben Curtis, Rodney Muhumuza and Jacob Kushner in Nairobi contributed to this report.

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Follow Adam Schreck on Twitter at http://www.twitter.com/adamschreck

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  • A plume of black smoke billows over the Westgate Mall, in Nairobi, Kenya Monday, Sept. 23, 2013. Multiple large blasts have rocked the mall where a hostage siege is in its third day. Associated Press reporters on the scene heard multiple blasts and a barrage of gunfire. Security forces have been attempting to rescue an unknown number of hostages inside the mall held by al-Qaida-linked terrorists.(AP Photo/Jonathan Kalan)

  • Armed police from the General Service Unit take cover behind a wall during a bout of gunfire, outside the Westgate Mall in Nairobi, Kenya Monday, Sept. 23, 2013. Hostages being held by al-Qaida-linked terrorists in a Nairobi mall have not been released despite an earlier statement from the military that "most" had been rescued, a person connected to the situation told The Associated Press on Monday. (AP Photo/Ben Curtis)

  • Paramedics and police officers run outside the Westgate Mall in Nairobi after heavy shooting started Monday, Sept. 23, 2013. Kenya's military launched a major operation at the upscale Nairobi mall and said it had rescued "most" of the hostages being held captive by al-Qaida-linked militants during the standoff. (AP Photo/Sayyid Azim)

  • Kenya security forces are seen behind a wall outside the Westgate Mall in Nairobi, Kenya Monday morning, Sept. 23, 2013. Kenya's military launched a major operation at the upscale Nairobi mall and said it had rescued "most" of the hostages being held captive by al-Qaida-linked militants during the standoff that killed at least 68 people and injured 175. (AP Photo/Jerome Delay)

  • Kenya security personnel take cover outside the Westgate Mall after shooting started inside the mall early Monday morning, Sept. 23, 2013. Kenya's military launched a major operation at the upscale Nairobi mall and said it had rescued "most" of the hostages being held captive by al-Qaida-linked militants during the standoff that killed at least 68 people and injured 175. (AP Photo/Sayyid Azim)

  • Kenya security forces are seen behind a wall outside the Westgate Mall in Nairobi, Kenya Monday morning, Sept. 23, 2013. Kenya's military launched a major operation at the upscale Nairobi mall and said it had rescued "most" of the hostages being held captive by al-Qaida-linked militants during the standoff that killed at least 68 people and injured 175. (AP Photo/Jerome Delay)

  • Kenya security forces are seen behind a wall outside the Westgate Mall in Nairobi, Kenya Monday morning, Sept. 23, 2013. Kenya's military launched a major operation at the upscale Nairobi mall and said it had rescued "most" of the hostages being held captive by al-Qaida-linked militants during the standoff that killed at least 68 people and injured 175. (AP Photo/Jerome Delay)

  • Onlookers gather on a hill to observe the Westgate Mall after a bout of heavy gunfire just after dawn in Nairobi, Kenya Monday, Sept. 23, 2013. Helicopters circling overhead, Kenya's military launched a major operation Sunday night at the upscale Nairobi mall and claimed it had rescued "most" of the hostages being held captive by al-Qaida-linked militants during a standoff that killed at least 68 people and injured 175. (AP Photo/Ben Curtis)

  • Relatives wait at the Nairobi City Mortuary following the mall attack in Kenya, Sunday, Sept. 22, 2013. Kenyan authorities said Islamic extremist attackers remain inside the upscale Kenyan shopping mall, holding an unknown number of hostages, after killing at least 39 and injuring 150 during Saturday's attack. (AP Photo/Khalil Senosi)

  • Groups of onlookers gather on a road looking down over the Westgate Mall in Nairobi, Kenya Sunday, Sept. 22, 2013. Multiple barrages of gunfire erupted Sunday morning from the upscale Kenyan mall where there is a hostage standoff with Islamic extremists nearly 24 hours after they attacked using grenades and assault rifles. (AP Photo/Ben Curtis)

  • A line of soldiers from the Kenya Defense Forces run in front of the Westgate Mall in Nairobi, Kenya Sunday, Sept. 22, 2013. Multiple barrages of gunfire erupted Sunday morning from the upscale Kenyan mall where there is a hostage standoff with Islamic extremists nearly 24 hours after they attacked using grenades and assault rifles. (AP Photo/Ben Curtis)

  • Soldiers from the Kenya Defense Forces carry a wounded colleague, following the sound of explosions and gunfire, out of the Westgate Mall in Nairobi, Kenya Sunday, Sept. 22, 2013. Associated Press journalists at the scene of a hostage standoff said a barrage of gunfire erupted from the upscale Kenyan mall Sunday morning and moments later two wounded Kenyan security forces were carried out of the mall. (AP Photo/Ben Curtis)

  • Trucks of soldiers from the Kenya Defense Forces arrive after dawn outside the Westgate Mall in Nairobi, Kenya Sunday, Sept. 22, 2013. Islamic extremist gunmen lobbed grenades and fired assault rifles inside Nairobi's top mall Saturday, killing dozens and wounding over a hundred in the attack. Early Sunday morning, 12 hours after the attack began, gunmen remained holed up inside the mall with an unknown number of hostages. (AP Photo/Ben Curtis)

  • Trucks of soldiers from the Kenya Defense Forces arrive after dawn outside the Westgate Mall in Nairobi, Kenya Sunday, Sept. 22, 2013. Islamic extremist gunmen lobbed grenades and fired assault rifles inside Nairobi's top mall Saturday, killing dozens and wounding over a hundred in the attack. Early Sunday morning, 12 hours after the attack began, gunmen remained holed up inside the mall with an unknown number of hostages. (AP Photo/Ben Curtis)

  • Armed special force aim their guns at the Westgate Mall in Nairobi, Kenya on Saturday, Sept. 21 2013, after gunmen threw grenades and opened fire during an attack that left multiple dead and dozens wounded. A witness to the attacks on the upscale shopping mall says that gunmen told Muslims to stand up and leave and that non-Muslims would be targeted(AP Photo/Khalil Senosi)

  • Civilians who had been hiding inside during the gun battle manage to flee from the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013. Gunmen threw grenades and opened fire Saturday, killing at least 22 people in an attack targeting non-Muslims at an upscale mall in Kenya's capital that was hosting a children's day event, a Red Cross official and witnesses said. (AP Photo/Jonathan Kalan)

  • A body is seen on the floor, below right, as smoke fills the air inside the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013. Gunmen threw grenades and opened fire Saturday, killing at least 22 people in an attack targeting non-Muslims at an upscale mall in Kenya's capital that was hosting a children's day event, a Red Cross official and witnesses said. (AP Photo/Jonathan Kalan)

  • A woman who had been hiding during the gun battle runs for cover after armed police, seen behind, enter the Westgate Mall in Nairobi, Kenya, Saturday, Sept. 21, 2013. Gunmen threw grenades and opened fire Saturday, killing at least 22 people in an attack targeting non-Muslims at an upscale mall in Kenya's capital that was hosting a children's day event, a Red Cross official and witnesses said. (AP Photo/Jonathan Kalan)

  • An armed police officer takes cover during a bout of gunfire outside the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013. Gunmen threw grenades and opened fire Saturday killing at least 22 people in an attack targeting non-Muslims at an upscale mall in Kenya's capital that was hosting a children's day event, a Red Cross official and witnesses said. (AP Photo/Ben Curtis)

  • People run from the Westgate Mall in Nairobi, Kenya.

    Gunmen threw grenades and opened fire during an attack that left multiple dead and dozens wounded. A witness to the attacks on the upscale shopping mall says that gunmen told Muslims to stand up and leave and that non-Muslims would be targeted.

  • A woman who had been held hostage reacts after being released.

    Suspected terrorists engaged Kenyan security forces in a drawn out gun fight at the Westgate Mall in Nairboi.

  • Woman runs for cover.

    A woman who had been hiding during the gun battle runs for cover after armed police, seen behind, enter the Westgate Mall in Nairobi, Kenya, Saturday, Sept. 21, 2013.

  • Location of the attack.

    Graphic shows detail of shopping mall site of fatal gun attack in Nairobi, Kenya.

  • View inside the mall.

    Armed police take cover behind escalators as smoke fills the air inside the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013.

  • Raw: Gunmen Attack Shopping Mall in Kenya

    Gunmen threw grenades and opened fire at an upscale mall in Kenya's capital on Saturday, killing at least 10 people.

  • Armed police on scene.

    Armed police leave after entering the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013.

  • Mall-goers flee after attack.

    Civilians who had been hiding inside during the gun battle manage to flee from the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013.

  • An armed police officer takes cover during a bout of gunfire outside the Westgate Mall.

    The mall in Kenya's capital was hosting a children's day event at the time of the attack, a Red Cross official and witnesses said.

  • Armed police at the Westgate Mall.

    Armed police crouch down and take position during a gun battle outside the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013.

  • Civilians leave the mall after gun battle.

    Civilians who had been hiding during the gun battle hold their hands in the air as a precautionary measure before being searched by armed police leading them to safety, inside the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013.

  • A rescue worker helps a child outside the Westgate Mall.

  • Security helps a woman outside the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013, after gunmen threw grenades and opened fire during an attack that left multiple dead and dozens wounded.

  • People flee the scene of the attack.

    Security helps people outside the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013, after gunmen threw grenades and opened fire during an attack that left multiple dead and dozens wounded.

  • A wounded man is escorted away from the mall.

    Witnesses say a half dozen grenades also went off along with volleys of gunfire in and around the mall.

  • A rescued hostage collapses on the ground after being freed by police at Westgate Mall.

    A witness to the attacks on the upscale shopping mall says that gunmen told Muslims to stand up and leave and that non-Muslims would be targeted.

  • Two women hug outside the mall after the attack.

    Bursts of gunfire were heard from outside the Westgate Mall Saturday, where cars were left abandoned. Witnesses say a half dozen grenades also went off along with lobbies of gunfire that started at midday.

  • Woman escapes from the Westgate Mall.

    A woman who had been hiding inside and managed to flee, sits in a state of shock, as an unidentified armed member of the security forces, above, keeps watch outside the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013.

  • Security officers keep a crowd under control outside the Westgate Mall

  • An injured person makes a phone call as he is helped on his arrival at the Aga Khan Hospital after the attack at the Westgate Mall.

  • An injured person arrives at the hospital after the attack on the Westgate Mall.

  • A injured child is wheeled away on a shopping trolley outside the Westgate Mall.

  • A security officer helps a wounded woman outside the Westgate Mall in Nairobi, Kenya Saturday, Sept. 21, 2013, after gunmen threw grenades and opened fire during an attack that left multiple dead and dozens wounded. A witness to the attacks on the upscale shopping mall says that gunmen told Muslims to stand up and leave and that non-Muslims would be targeted. (AP Photo/Khalil Senosi)

  • An injured man arrives at the Aga Khan Hospital in Nairobi after the attack at the Westgate Mall.

  • An injured man arrives at the Aga Khan Hospital after the attack at the Westgate Mall.

    A witness to the attacks says that gunmen told Muslims to stand up and leave and that non-Muslims would be targeted. Initial police reports had described the incident as a botched robbery. Witnesses say a half dozen grenades also went off along with volleys of gunfire in and around the mall.

  • A security guard helps a child outside the Westgate Mall.

    A witness to the attacks on the upscale shopping mall says that gunmen told Muslims to stand up and leave and that non-Muslims would be targeted.