SOFIA, Bulgaria — A sophisticated group of conspirators was involved in the suicide bombing in Bulgaria that killed five Israeli tourists and a Bulgarian bus driver last week, and they spent about a month in Bulgaria before the attack, the country's prime minister said Tuesday.

Boiko Borisov's comments confirm suspicions that the suicide attacker who targeted a bus filled with Israeli vacationers last Wednesday did not act alone. However, the prime minister didn't say how many people were believed to have been involved in the attack on Israeli tourists in Burgas and also declined to back up Israel's claims that Iran and the militant group Hezbollah played roles.

Those involved used "leased vehicles, they moved in different cities so as not to be seen together, and no two of them can be seen in one place on any security camera," Borisov said, speaking alongside visiting White House counterterrorism chief John Brennan.

He described the people behind the blast as "exceptionally skilled" and said they "observed absolute secrecy." He also said DNA samples from the suicide bomber have been shared with all partner security services, but no match has been found in their databases.

"There was absolutely no chance of preventing such an act of violence," Borisov insisted. "We could have only detected it by chance or if we had been informed by the services that such activities were under way in Bulgaria."

He said officials believe he might have flow into Bulgaria from a European country in the Schengen passport-free travel zone, and that Bulgaria is exploring that lead with officials in other European countries. Though a European Union member, Bulgaria doesn't yet belong to the Schengen area.

"We do not know his identity, but it is known when he has arrived, the presumed flight, where he came from. It could turn out that he entered Bulgaria from a Schengen member country," Borisov said.

Brennan also stopped short of blaming Iran or Hezbollah, both of which are U.S. nemeses, though he noted both Tehran and the Lebanese group had been implicated in attacks on civilians in the past. He said the U.S. has been working with Bulgaria during its investigation.

"Bulgaria will continue to have the full support of the United States in the weeks and months ahead," Brennan said.

European security officials say that images of the attacker have not yet matched any of their databases but that it's possible the man was not on a watch list.

Investigators know, however, that he was using multiple aliases and was wearing a wig and disguise when he was captured on closed circuit television, according to a European security official who spoke on condition of anonymity because he was not authorized to speak about the investigation.

Israel's military chief insisted Tuesday that Iran and Hezbollah were involved and vowed that Israel would respond to the attack.

"We will have to find a way to respond to this attack, and not just a one-off," Lt. Gen. Benny Gantz was quoted as telling the Israeli parliament's foreign affairs and defense committee. "We will know how to do it judiciously. Ultimately, the response will come."

His comments were relayed by a meeting participant who discussed contents from the closed session on condition of anonymity.

Also Tuesday, Israeli and Bulgarian officials held a memorial ceremony at Burgas Airport to pay tribute to the victims of the attack with a minute of silence and the playing of the national anthems. Israeli Tourism Minister Stas Misezhnikov laid a wreath.

Misezhnikov said that some 180,000 Israeli tourists visit Bulgaria each year, and that they will continue coming in the future as well.

Since the attack, however, thousands of Israelis have already canceled plans to vacation in Bulgaria this summer.

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Associated Press writers Paisley Dodds in London and Amy Teibel in Jerusalem contributed to this report.

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  • A Israeli survivor from a bombing in Bulgaria that killed seven people on Wednesday, cries as he arrives at the Ben Gurion airport in Tel Aviv, Israel, Thursday, July, 19, 2012. (AP Photo/Dan Balilty)

  • Israeli women whose relatives were among the victims of a bombing in Bulgaria that killed seven people on Wednesday, July 18, 2012 wait for their arrival at the Ben Gurion airport in Tel Aviv, Israel, Thursday, July, 19, 2012. (AP Photo/Dan Balilty)

  • Relatives mourn Maor Harush who was killed in a suicide bombing in Bulgaria during his funeral in Acco, Israel, Friday, July 20, 2012. (AP Photo/Ahikam Seri)

  • Relatives mourn Maor Harush who was killed in a suicide bombing in Bulgaria during his funeral in Acco, Israel, Friday, July 20, 2012. (AP Photo/Ahikam Seri)

  • Family and friends attend the funeral of Itzik Kolengi, 28, who was killed and his wife injured in a suicide bombing in Bulgaria Wednesday in Petah Tikva, Israel, Friday, July 20, 2012. (AP Photo/Dan Balilty)

  • Relatives mourn Maor Harush who was killed in a suicide bombing in Bulgaria during his funeral in Acco, Israel, Friday, July 20, 2012. (AP Photo/Ahikam Seri)

  • People carry the body of Itzik Kolengi, 28, who was killed and his wife injured in a suicide bombing in Bulgaria Wednesday in Petah Tikva, Israel, Friday, July 20, 2012. (AP Photo/Dan Balilty)

  • Family and friends attend the funeral of Itzik Kolengi, 28, who was killed and his wife injured in a suicide bombing in Bulgaria Wednesday in Petah Tikva, Israel, Friday, July 20, 2012. (AP Photo/Dan Balilty)

  • Relatives cry over a coffin of victims killed in bombing in Bulgaria after the remains arrived at Tel Aviv airport, Israel, Friday, July, 20, 2012. (AP Photo/Dan Balilty)

  • Israeli soldiers carry the coffins of people killed in a bombing in Bulgaria as the remains arrived back at an airport in Tel Aviv, Israel, Friday, July 20, 2012. (AP Photo/Dan Balilty)

  • Israeli soldiers carry the coffins of people killed in a bombing in Bulgaria as the remains arrived back at an airport in Tel Aviv, Israel, Friday, July 20, 2012. (AP Photo/Dan Balilty)

  • Israeli soldiers carry a coffin of victims who were killed in bombing in Bulgaria after the remains arrived at Tel Aviv ariprt, Israel, Friday, July, 20, 2012. (AP Photo/Dan Balilty)

  • Israeli soldiers carry a coffin of victimes killed in bombing in Bulgaria after the remains arrived at Tel Aviv airport, Israel, Friday, July, 20, 2012. (AP Photo/Dan Balilty)

  • Relatives react as the coffins of people killed in a bombing in Bulgaria arrived back at an airport in Tel Aviv, Israel, Friday, July 20, 2012. (AP Photo/Dan Balilty)

  • An Israeli family cries during a military ceremony for the victims who were killed in an attack in Bulgaria, at Tel Aviv airport, Israel, Friday, July, 20, 2012. (AP Photo/Dan Balilty)

  • Relatives sit in front of the coffins of people killed in bombing in Bulgaria after the remains arrived at Tel Aviv airport, Israel, Friday, July, 20, 2012. (AP Photo/Dan Balilty)

  • This image taken from security video provided by the Bulgarian Interior Ministry Thursday, July 19, 2012 purports to show the unidentified bomber, center, with long hair and wearing a baseball cap, at Burgas Airport in Burgas, Bulgaria, on Wednesday, July 18, 2012. (AP Photo/Bulgarian Interior Ministry)

  • This image taken from security video provided by the Bulgarian Interior Ministry Thursday, July 19, 2012 purports to show the unidentified bomber, center, with long hair and wearing a baseball cap, at Burgas Airport in Burgas, Bulgaria, on Wednesday, July 18, 2012. (AP Photo/Bulgarian Interior Ministry)

  • Israelis move through the departure terminal at Ben-Gurion International Airport in Tel Aviv, Thursday, July, 19, 2012. (AP Photo/Dan Balilty)

  • Smoke rises into the sky after an explosion at Burgas airport, outside the Black Sea city of Burgas, Bulgaria, some 400 kilometers (250 miles) east of the capital, Sofia, Wednesday, July 18, 2012. A bus carrying young Israeli tourists in a Bulgarian resort exploded Wednesday, killing three people and wounding at least 20, police said. Witnesses told Israeli media that the huge blast occurred soon after someone boarded the vehicle. (AP Photo/ Burgasinfo) BULGARIA OUT

  • An unidentified injured Israeli tourist is carried in front of Borgas hospital after an explosion at Burgas airport, outside the Black Sea city of Burgas, Bulgaria, some 400 kilometers (250 miles) east of the capital, Sofia, Wednesday, July 18, 2012. (AP Photo/ Bulphoto Agency) BULGARIA OUT

  • Unidentified Israeli tourist is helped as she arrives to Bourgas hospital after a bus carrying Israeli tourists in the Bulgarian resort city of Bourgas exploded Wednesday, July 18, 2012, killing at least three people and wounding more than 20 others, police said. (AP Photo/ Bulphoto Agency)

  • An Israeli woman wounded in a bombing in Bulgaria that left five Israeli tourists dead on Wednesday is comforted by a medic at Soroka hospital in the southern city of Beersheva, Israel, Thursday, July 19, 2012. (AP Photo/Tsafrir Abayov)

  • A damaged bus is transported out of Burgas airport, Bulgaria, Thursday, July 19, 2012 a day after a deadly suicide attack on a bus full of Israeli vacationers. (AP Photo/ Impact Press Group)

  • A damaged bus is transported out of Burgas airport, Bulgaria, Thursday, July 19, 2012 a day after a deadly suicide attack on a bus full of Israeli vacationers. (AP Photo/ Impact Press Group)

  • A Jewish man reads prayers during a mourning ceremony at a synagogue in Sofia, Thursday, July 19, 2012. (AP Photo/Valentina Petrova)

  • Bulgaria's Interior Minister Cvetan Cvetanov, center, speaks during a press conference at Burgas airport, Bulgaria, Thursday, July 19, 2012 a day after a deadly suicide attack on a bus full of Israeli vacationers. (AP Photo)

  • Bulgaria's Interior Minister Cvetan Cvetanov, center, speaks during a press conference at Burgas airport, Bulgaria, Thursday, July 19, 2012 a day after a deadly suicide attack on a bus full of Israeli vacationers. (AP Photo)