NAIROBI, Kenya — An explosion ripped through a building full of small shops in downtown Nairobi on Monday, injuring at least 33 people, including a woman who blamed the blast on a "bearded man" who left behind a bag shortly before the detonation.

Police officials first indicated the explosion could have been caused by some sort of electrical malfunction but the prime minister said it was deliberate. Al-Shabab – an Islamist militant group from Somalia – has threatened to carry out such an attack.

"This is a heinous act," Prime Minister Raila Odinga said while visiting the scene of the blast. "They want to scare us. But we will not be scared."

The explosion sent dark smoke billowing out of a one-story building on Moi Avenue, named after Kenya's second president. The blast peeled back the front corner of the building's aluminum roof, shattered windows in the building and scattered shoes, clothes and other wares on the ground. A high-rise building with a glass exterior next door was largely untouched.

Speaking to The Associated Press from a Nairobi hospital bed, Irene Wachira said a bearded man came to a nearby stall three times and acted as if he were interested in buying something. Wachira said the third time he came with a bag that he left behind. The blast occurred shortly afterward, she said.

Wachira, a vendor in the building, described the man as "Arabic-looking" because of his relatively light skin. A doctor told AP that another person wounded in the blast said a Somali-looking man left behind the bag. The doctor said he could not be quoted by name.

Police officials who first responded hesitated to blame terrorism, given the lack of shrapnel. Kenya Power ruled out an electrical fault as the cause. The national electricity agency said the building had no ground-mounted transformer that would explode and determined that all electrical connections to the building that would blow in a short circuit remained intact.

The police later released a statement saying that the cause of the explosion had not yet been established. Police are investigating the possibility that an improvised explosive device caused the blast, though the police said it was unlikely a conventional bomb had been used.

After the explosion, bloodied people received medical care on the street as authorities tried to usher hundreds of people away. The scene was played out just a few blocks from where the U.S. Embassy had been destroyed by a truck bomb in 1998. Al-Qaida's near-simultaneous attacks on the U.S. embassies in Nairobi and Dar es Salaam, Tanzania, killed 224 people.

Odinga said security would be improved downtown and that the Somali militants, who are linked to al-Qaida, "want to scare investors. They want to scare tourists."

"We condemn the terrorists and tell them their days are numbered," said Odinga, who is expected to run for president in next year's election.

Al-Shabab threaten to carry out large-scale attacks here followed Kenya's decision last October to send troops into Somalia to pursue the Islamist militants. Kenya said back then that it was sending in the troops as a response to kidnappings on Kenyan soil last year blamed on al-Shabab. The kidnappings caused tourism in Kenya to plummet, especially around the coastal resort of Lamu.

Since October, a series of grenade attacks has rocked Kenya. The latest happened Saturday night in the sprawling Dadaab refugee camp near the Somali border. Six people were injured in two simultaneous grenades blasts, officials said.

The police said at least 33 people were wounded in Monday's explosion, including five with serious injuries such as burns, fractures and deep lacerations. None of the victims suffered shrapnel wounds, said Thomas Mutie, the acting chief executive at Kenyatta National Hospital.

Earlier on HuffPost:

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  • Kenyan Firefighters and Kenyan plain clothes police officer at a shop after the explosion on a busy road in Nairobi, Kenya, Monday, May 28, 2012. An explosion ripped through a building, full of small shops in downtown Nairobi on Monday, wounding an unknown number of people. The force of the blast ripped apart the one-storey building's aluminum roof, but a high-rise building with a glass exterior right next to the blast did not appear to sustain major damage. There was no immediate word on casualties or what caused the blast. (AP Photo/Sayyid Azim)

  • A plain clothed police officer reacts as fire officers sort through the debris of a building after an explosion in Nairobi, Kenya, Monday, May 28, 2012. (AP Photo/ Khalil Senosi)

  • Medical personnal at Kenyatta National Hospital in Nairobi, Kenya, attend people injured in an explosion Monday, May 28, 2012. (AP Photo/ Khalil Senosi)

  • Kenyan firefighters work in shop after an explosion on a busy street in downtown Nairobi, Kenya, Monday, May 28, 2012. (AP Photo/Sayyid Azim)

  • Clothes and shoes are scattered outside a shop after an explosion on a busy street in downtown Nairobi, Kenya, Monday, May 28, 2012. (AP Photo/Sayyid Azim)

  • An investigator stands amid shoes and clothes scattered outside a shop after an explosion on a busy street in downtown Nairobi, Kenya, Monday, May 28, 2012. (AP Photo/Sayyid Azim)

  • Kenyan firefighter looks through the wreckage of a shop after the explosion on a busy road in downtown Nairobi, Kenya, Monday, May 28, 2012. (AP Photo/Sayyid Azim)

  • Kenyan paramilitary personnel control the large crowd at the scene of the explosion on a busy road in downtown Nairobi, Kenya, Monday, May 28, 2012. (AP Photo/Sayyid Azim)