The suicide bomber who killed six people in Bulgaria last week was part of a sophisticated group of conspirators, Prime Minister Boiko Borisov has said.
He said the group had arrived in Bulgaria a month before the attack.
But he declined to back Israeli claims that Iran and Lebanese militant group Hezbollah played a role.
Five Israeli tourists and a Bulgarian bus driver died in last Wednesday's bombing in Burgas on the Black Sea.
The identity of the man who carried out the bombing remains unclear.
As well as those killed, dozens of people were wounded in the blast.
Israeli Tourism Minister Stas Misezhnikov laid a wreath at the bomb site during a memorial ceremony on Tuesday.
Mr Borisov, who was speaking in Sofia alongside White House counter-terrorism chief John Brennan, said the US was supporting Bulgaria's investigation into the attack.
Mr Borisov did not say exactly how many people had been involved in the conspiracy he described, but he said they had been "exceptionally skilled" and operated under "strict conspiracy rules" to keep the plot hidden.
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".
"There was absolutely no chance of preventing such an act of violence," the prime minister 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."
Shortly after the bombing, Bulgarian authorities released CCTV images of the man they believed carried out the bombing, but they appeared to contrast with some witness descriptions.
Mr Borisov said authorities had shared fingerprints and DNA samples with other security services but no match had been found, and the man's identity remained elusive.
But he added that they knew "when he arrived, the presumed flight and where it came from", reported AFP news agency.
Mr Borisov also refused to back Israeli claims of involvement by Iran or Hezbollah, saying "we do not want to get involved in this long-standing conflict as we are very vulnerable".