SOFIA (Reuters) - A suicide bomber who killed five Israeli tourists when he blew up a bus in Bulgaria last week was backed up by an organized group who helped him plan and carry out the attack, Boiko Borisov, the Bulgarian prime minister, said on Tuesday.
Borisov said police had not yet identified the bomber whose attack also wounded more than 30 people at Burgas airport last Wednesday, but said the man had not acted alone.
"These are extremely experienced people who have followed strict conspiracy rules," Borisov told reporters after meeting John Brennan, a counter-terrorism adviser to U.S. President Barack Obama.
"From what we see, they arrived nearly a month beforehand, changed rental cars, and travelled to different cities ... and not more than one of the people we are looking for was captured on either security camera," Borisov said.
He declined to give more details on the plotters.
Borisov said that the bomber's DNA and finger prints had not matched anything held on file by Bulgaria or by partner spy agencies and that police were still working to identify him.
But he suggested that the attacker, whose bomb was concealed in his backpack, may have entered Bulgaria on a plane from the European Union's "Schengen" passport-free travel zone. He did not elaborate.
Israel has accused Iran and the Lebanese Islamist group Hezbollah of the bombing. Iran has denied the accusations.
Borisov said that Bulgaria - a member of both the EU and NATO - would not say who it thought was responsible for the attack until the investigation was complete.
(Reporting by Tsvetelia Tsolova; Editing by Andrew Osborn)