A sophisticated group of conspirators were involved in the suicide bombing that killed five Israelis and a Bulgarian, and they spent at least a month in Bulgaria before the attack, the country's prime minister said Tuesday.

Boiko Borisov's comments confirmed suspicions that the suicide attacker who targeted a bus filled with Israeli vacationers did not act alone. However, the prime minister did not give many details - including how many people were believed to have been involved in the Wednesday attack. He also declined to back up Israel's claims that Iran and 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 alongside visiting White House counterterrorism chief John Brennan.

He described the people behind the blast "exceptionally skilled" and said they operated under "strict conspiracy rules." The prime minister 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 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."

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. ¬supports Bulgaria's efforts to track down those responsible and urged authorities to gather as much evidence as possible.

The suicide attack occurred in the Black Sea resort town of Burgas. Along with the five vacationing Israelis and Bulgarian bus driver who were killed, dozens of other people were wounded.

Bulgaria, an eastern European nation of 7.3 million, has been a popular travel destination in recent years for Israelis.