SOFIA, Bulgaria -- The Bulgarian government said on Tuesday that two of the people behind a deadly bombing attack that targeted an Israeli tour bus six months ago were believed to be members of the military wing of the Lebanese militant group Hezbollah.
The announcement could force the European Union to reconsider whether to designate the group as a terrorist organization and crack down on its extensive fund-raising operations across the continent. That could have wide-reaching repercussions for Europe's uneasy détente with the group, which is an influential force in Middle East politics, considers Israel an enemy and has extensive links with Iran.
Bulgaria's interior minister, Tsvetan Tsvetanov, said at a news conference that the investigation into the bombing in Burgas in July 2012 found that a man with an Australian passport and a man with a Canadian passport were two of the three conspirators involved in the attack, which claimed the lives of five Israeli tourists and a Bulgarian bus driver.
Bulgarian investigators had "a well-founded assumption that they belonged to the military formation of Hezbollah," Mr. Tsvetanov said.
Bulgarian officials have found themselves under pressure from Israel and the United States, which consider Hezbollah a terrorist organization, to blame it for the bus attack. But the Bulgarians also have been facing pressure from European allies like Germany and France, which regard Hezbollah as a legitimate political organization, to temper any finding on the sensitive issue.
The United States welcomed the finding. "We call on our European partners as well as other members of the international community to take proactive action to uncover Hizballah's infrastructure and disrupt the group's financing schemes and operational networks in order to prevent future attacks," said John O. Brennan, President Obama's chief counterterrorism adviser and his nominee to run the Central Intelligence Agency, in a statement Tuesday.
But Catherine Ashton, the European Union's high representative for foreign policy, responded with caution. "The implications of the investigation need to be assessed seriously as they relate to a terrorist attack on E.U. soil, which resulted in the killing and injury of innocent civilians," she said in a statement.
The new secretary of state, John Kerry, released a statement urging "governments around the world – and particularly our partners in Europe – to take immediate action to crack down" on Hezbollah, and made a phone call to Ms. Ashton. Asked if he had pressed for Hezbollah to be blacklisted, the State Department spokeswoman, Victoria Nuland, said that Ms. Ashton "knows where we want to go."
An E.U. official, speaking on condition of anonymity due to the sensitivity of the issue, said that the question of listing Hezbollah as a terrorist organization would have to be a unanimous decision by all 27 member states.
Mr. Tsvetanov spoke to reporters here after briefing top government officials and security personnel about the state of the investigation.
"We have followed their entire activities in Australia and Canada so we have information about financing and their membership in Hezbollah," he said.
Mr. Tsvetanov did not mention Iran, however, Hezbollah's ally and chief backer.
The Israeli prime minister, Benjamin Netanyahu, however, drew a direct link. "This is yet a further corroboration of what we have already known, that Hezbollah and its Iranian patrons are orchestrating a worldwide campaign of terror that is spanning countries and continents," Mr. Netanyahu said, according to a statement released by his office.
A senior Israeli official, speaking on condition of anonymity in advance of Mr. Netanyahu's statement, said Jerusalem was pleased with the Bulgarian report, which he said would "make it much more difficult" for European countries "to circumvent debate about the true nature of Hezbollah."
The Israeli official, along with an Israeli counter-terrorism expert, said that it was not surprising that Iran was not mentioned, because Bulgaria's investigation did not extend beyond its own borders, and was focused on what happened on the ground, not the larger question of who approved or financed the operation.
"The fact that they didn't put their finger in front of Iran and leading to the responsibility of Iran, one cannot blame them," said Boaz Ganor, head of the Institute for Counter-Terrorism at the Interdisciplinary Center in Herzilya, Israel. "You need a different level of intelligence, a different level of facts in your hands, to prove the next level, which is that Hezbollah was conducting that under the initiation or the approval of Iran. That they probably don't have right now."
Hezbollah has denied responsibility for the bombing. Amin Hotait, a retired general in the Lebanese army close to Hezbollah, said the decision "Lacks the unequivocal evidence."
"The party doesn't usually retaliate against Israeli attacks by killing civilians," Mr. Hotait said. "This decision is political in nature, since Bulgaria is not independent country, but politically dependent on the West."
But analysts said the bombing was one chapter in a shadow war pitting Israel against Iran and Hezbollah. Israel is believed to be behind the killings of Iranian nuclear scientists. Operatives of the Iranian Quds Force, an elite international operations unit within Iran's Islamic Revolutionary Guards Corps, were believed to be behind a series of plots against Israeli targets in Thailand, India, Georgia and elsewhere. Israeli officials said the Burgas attack bore the hallmarks of a Hezbollah operation.
The European calculation all along has been that whatever its activities in the Middle East, Hezbollah does not pose a threat on the Continent. Thousands of Hezbollah members and supporters operate in Europe essentially unrestricted, raising money that is funneled back to the group in Lebanon.
Changing the designation to a terrorist entity raises the prospect of unsettling questions for Europe -- how to deal with those supporters, for example -- and the sort of confrontation governments have sought to avoid.
"There's the overall fear if we're too noisy about this, Hezbollah might strike again, and it might not be Israeli tourists this time," said Sylke Tempel, editor in chief of the German foreign affairs magazine Internationale Politik.
Bulgarian officials would like to maintain strong ties with Israel and the United States, and European allies like France and Germany. They had maintained a studied silence for more than six months since the attack.
"If you factor in the suspicion that there are political implications beyond Bulgaria's borders, it's completely understandable that they've been playing for time," said Dimitar Bechev, head of the Sofia office of the European Council on Foreign Relations.
Mr. Tsvetanov spoke after the meeting of the president's council for national security, which includes the prime minister, top cabinet members and military and security personnel.
Bulgarian officials are acutely aware of the consequences of their findings even though larger European Union members did not exert blatant pressure on them regarding the Hezbollah question. "It was not a campaign," said Philipp Missfelder, a leading member of Chancellor Angela Merkel's Christian Democrats and the foreign policy spokesman for the party in Parliament. "Some German officials dropped a few words."
But Mr. Missfelder said that attitudes toward Hezbollah were gradually shifting. "It's clear that they are steered from Iran and they are destabilizing the region," Mr. Missfelder said. "The group that thinks Hezbollah is a stabilizing factor is getting smaller."
Hezbollah's dual nature as what Western intelligence agencies call a terrorist organization and a political party with significant social projects, including schools and health clinics, make it more difficult to dismiss. Hezbollah is a significant political actor in Lebanon, and many European officials are particularly wary of upsetting the status quo as the civil war drags on in Syria.
A sort of modus vivendi exists where Hezbollah keeps a low profile for its fund-raising and other activities and Europeans do not crack down. In Germany alone, 950 people have been identified as being associated with the organization as of 2011. The group has always been treated as a benign force, even if assessments of the danger it presented vary greatly.
The senior Israeli official said Jerusalem was pleased with the Bulgarian report, which he said would "make it much more difficult" for European countries "to circumvent debate about the true nature of Hezbollah."
"We quickly came to the conclusion that Hezbollah was behind it," the official said. "The Bulgarians wanted to dig deeper before they were willing to say in public what they found. And who that led to. They dug deeper and deeper, and the deeper they dug, the closer they got to Hezbollah."
The official said that Israel had shared intelligence reports with the Bulgarian authorities and that the Bulgarian investigators had briefed Jerusalem about their findings, but that Israel purposely kept its distance because an independent report from a European country would be more powerful. "We didn't know what they were going to say," he said. "It was up to them to decide how they want to play this politically."
Reporting was contributed by Jodi Rudoren from Jerusalem, Hwaida Saad from Beirut, and Eric Schmitt and Michael Gordon from Washington.
This article originally appeared in The New York Times.