The suicide bomber who killed six people in an attack on Israelis had accomplices and may have entered Bulgaria from Europe's Schengen passport-free area, Prime Minister Boyko Borisov said Tuesday.
"It might turn out he came from a Schengen member state. This is a lead we are checking at the moment with several other EU partner services," Borisov said.
He said the bomber and his suspected accomplices behind last Wednesday's attack at a Black Sea airport which Israel has blamed on Iran and the Lebanese militia Hezbollah were "extremely experienced".
Five Israelis and a Bulgarian driver were killed when the attacker blew himself up on a bus packed with Israelis at Burga airport, the first attack of its kind on Bulgarian soil.
"From what we can see, they came a month in advance," Borisov said at a press conference of the alleged bombing team.
"They changed hire car again and again. They stayed in different cities so that they would not be seen together -- no camera footage shows more than one person from the ones we are looking for."
"These are extremely experienced people who observed absolute secrecy," he added after talks with US President Barak Obama's Assistant for Homeland Security and Counterterrorism, John Brennan.
The way the attack was done "there was no stopping it" unless the Bulgarian services had "come by chance upon the explosive while it was being prepared," he said, suggesting the bomb was most probably made in Bulgaria.
Borisov also said investigators have been unable to match fingerprints or DNA samples taken from the bomber with databases around the world.
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"We do not know categorically his identity," he said, adding however: "We know when he arrived, the presumed flight and where it came from".
Soon after the bombing, Bulgarian authorities released closed circuit television footage from the airport of a young man in typical tourist gear and long hair whom they believe was the suicide bomber.
Speaking to AFP on Monday, Galina Mileva, the coroner who examined the attacker's remains, put his age at "between 25 and 30-something" and said the bomber had "fair skin" but could have been of Arab origin.
Bulgaria is a member of the European Union but is not part of the borderless Schengen travel area formed by 22 of the 27 EU states plus non-EU Iceland, Liechtenstein, Norway and Switzerland.
The European Union turned down a request from Israel Tuesday to blacklist Hezbollah as a terror group in the wake of the bombing.
Israel has blamed Iran and its "terrorist proxy" Hezbollah but Brennan and Borisov stressed on Tuesday that there was no concrete proof of this so far. Iran has denied any involvement.
"The US is very concerned about the activities of Hezbollah as well as the activities of Iran in the terrorism realm... But again we will want to see the results of the Bulgarian investigation," Brennan said.
Borisov added: "We do not want to get involved in this long-standing conflict as we are very vulnerable... We cannot allow ourselves to point fingers at anyone."
Israeli Tourism Minister Stas Misezhnikov travelled to Burgas Tuesday to lay flowers at the attack site and is due to meet Israeli tourists and tour operators in Burgas and Varna, another favourite summer spot for Israelis.
Some 140,000 Israeli tourists visited Bulgaria last year and tour operators had hoped ahead of the attack for a substantial rise this year but are now crossing their fingers that the blow from the attack will not be too hard.