Tsvetanov also did not address the mystery of whether the attack was a suicide bombing. The explosive device carried a mechanism enabling it to be detonated from afar, according to the European counterterror official. Some Western investigators believe the bomber did not intend to die, but rather that an accomplice set off the bomb prematurely, in response to unforeseen circumstances or because the bomber was a dupe.
"The discovery of his license makes it more likely that he was not a suicide bomber," the European counterterror official said. "If it had all been planned to happen exactly as it did with his knowledge, I think they would have taken the license away to make it harder to trace him. We think it is an attacker who died rather than a voluntary kamikaze."
Although Hezbollah has conducted suicide car bombings against hard targets, it generally does not carry out lone-bomber suicide attacks on foot, said Magnus Ranstorp, a foremost Hezbollah expert at the Swedish National Defense College.
Hezbollah has used unwitting bombers before, he said, adding that there are "odd things" about the attack that have yet to be explained, such as the clumsily forged licenses.
But he and other experts said the timing, target and profile of the suspects suggests the attack was part of an escalating offensive by Iranian spies and Hezbollah militants that has resulted in attacks and plots from Africa to India to Thailand.
The shadow war has intensified because of Israeli assassinations of Iranian nuclear scientists and of Imad Mughniyeh, a Hezbollah warlord with close ties to the Iranian security forces, in 2008. The Burgas attack happened on the 18th anniversary of a massive car-bomb in Argentina carried out by Iranian and Hezbollah operatives in 1994.
Although Europe supports U.S. and Israeli efforts to thwart Tehran's nuclear ambitions, the European Union's policy toward Hezbollah diverges considerably. Most European nations resist pressure to designate Hezbollah, even its military wing, as a terrorist organization. In a clear sign of that reluctance, EU foreign minister Catherine Ashton issued a response to the Bulgarian announcement Tuesday that did not use the word Hezbollah.
Even European Union nations that view Hezbollah as a threat worry that designation could spur retaliation and destabilize Lebanon, where the militant group wields considerable power in government and on the street, Ranstorp said. European leaders also worry about the complex conflict with Iran involving Syria's civil war and the Iranian nuclear program.
"It's not a stand-alone issue," he said. "It's wrapped up in what's happening with Syria, Israel and above all, Iran."