SOFIA, Bulgaria — Hezbollah bombed a bus filled with Israeli tourists in Bulgaria last year, investigators said Tuesday, describing a sophisticated attack carried out by a terrorist cell that included Canadian and Australian citizens.

Bulgarian Interior Minister Tsvetan Tsvetanov, in the first major announcement in the investigation into the July 18 bombing that killed five Israelis and their Bulgarian driver, said one of the suspects entered the country with a Canadian passport, and another with one from Australia.

“We have well-grounded reasons to suggest that the two were members of the militant wing of Hezbollah,” Tsvetanov said after a meeting of Bulgaria’s National Security Council.

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A third suspect entered Bulgaria with them on June 28, he said, without giving details.

“We expect the government of Lebanon to assist in the further investigation, said Tsvetanov”

Hezbollah, a Shiite terrorist group and political party, has been linked to attacks and kidnappings on Israeli and Jewish interests around the world. The group has denied involvement in the Bulgaria bombing, and Hezbollah officials in Beirut declined further comment Tuesday. Hezbollah officials as a rule leave it to their leader, Sheikh Hassan Nasrallah, to comment on security issues.

The bomb exploded as the bus took a group of Israeli tourists from the airport to their hotel in the Black Sea resort of Burgas. The blast also killed the suspected bomber, a tall and lanky pale-skinned man wearing a baseball cap and dressed like a tourist.

In an interview with The Associated Press, Europol Director Rob Wainwright said the bomb was detonated remotely using a circuit board that a Europol expert has analyzed. Although it was initially believed to be a suicide bombing, Wainwright said investigators believe the bomber never intended to die.

Two counterfeit US driver’s licenses that were found near the bombing scene were traced back to Lebanon, where they were made, Wainwright said.

He said forensic evidence, intelligence sources and patterns in past attacks all point to Hezbollah’s involvement in the blast.

“The Bulgarian authorities are making quite a strong assumption that this is the work of Hezbollah,” Wainwright said. “From what I’ve seen of the case — from the very strong, obvious links to Lebanon, from the modus operandi of the terrorist attack and from other intelligence that we see — I think that is a reasonable assumption.”

Europol, which helps coordinate national police across the 27-nation European Union, which includes Bulgaria, sent several specialists to help investigate the attack.

The investigators found no direct links to Iran or to any al-Qaeda-affiliated terror group, Wainwright said.

The Bulgarian Foreign Ministry said security around the Lebanese Embassy was tightened following the announcement. Lebanon’s Prime Minister Najib Mikati condemned the attack and said his country would cooperate fully in its investigation.

Prime Minister Benjamin Netanyahu thanked the Bulgarian government for its thorough and professional investigation. “The Bulgarian findings announced today are clear: Hezbollah was directly responsible for the atrocity. There is only one Hezbollah. It is one organization with one leadership,” said Netanyahu. “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.”

Netanyahu added, “We hope the Europeans draw the necessary conclusions regarding the true nature of the Hezbollah.”

In strongly worded statements, Secretary of State John Kerry and White House counterterrorism adviser John Brennan said the Europeans, along with other countries that have balked at imposing sanctions on Hezbollah, must act to prevent additional attacks.

“We strongly urge other governments around the world — and particularly our partners in Europe — to take immediate action to crack down on Hezbollah,” Kerry said. “We need to send an unequivocal message to this terrorist group that it can no longer engage in despicable actions with impunity.”

Brennan, who is President Barack Obama’s nominee to run the Central Intelligence Agency, said the Bulgarian investigation “exposes Hezbollah for what it is: a terrorist group that is willing to recklessly attack innocent men, women, and children, and that poses a real and growing threat not only to Europe, but to the rest of the world.”

US officials also repeated the long-standing US position that Washington wants the EU to designate Hezbollah a terrorist organization.

Canadian Foreign Affairs Minister John Baird went further.

“We urge the European Union and all partners who have not already done so to list Hezbollah as a terrorist entity and prosecute terrorist acts committed by this inhumane organization to the fullest possible extent,” he said, adding that Canada would work with Bulgarian authorities given the apparent involvement of “a dual national living in Lebanon.”

“It is important that the EU respond robustly to an attack on European soil,” read a statement by British Foreign Secretary William Hague. “Every act of terror is an attack on our shared values. In committing an attack, terrorists seek to undermine our resolve, but they should only serve to strengthen it. The Home Secretary and I will be talking to our EU colleagues about the measures we can now take to continue to make our citizens safer.”

Catherine Ashton, the European Union’s high representative for foreign and security policy, said the EU needs to assess the implications of the investigation seriously but stressed that any decision on adding Hezbollah to the EU list of terrorist organizations would require a unanimous decision by the foreign ministers of the 27 EU countries, whose next scheduled meeting is Feb. 18. Such a move would freeze Hezbollah assets and cut off funding.

France and Germany, wary of coming under increased pressure to do so, had pressured investigators not to publicly name Hezbollah as responsible for the bombing, according to a U.S. official who spoke on condition of anonymity because he was not authorized to speak to the news media.

Linking Hezbollah to the attack is likely to escalate tensions between Israel and Iran, and provoke diplomatic headaches within Europe.

The United States considers Hezbollah a terrorist organization. The EU does not, and linking the group to the Bulgarian attack will increase pressure on it to do so. France and Germany had pressured investigators not to publicly name Hezbollah as responsible for the bombing, according to a US official who spoke on condition of anonymity because he was not authorized to speak to the news media.

For Hezbollah, the accusation comes at a horrible time.

Despite its formidable weapons arsenal and political clout in Lebanon, the group’s credibility and maneuvering space has been significantly reduced in recent years, largely because of the war in neighboring Syria but also because of unprecedented challenges at home.

Hezbollah still suffers from the fallout of a month-long 2006 war with Israel, in which it was blamed by many in the country for provoking an unnecessary conflict by kidnapping soldiers from the border area.

Since then, the group has come under increasing pressure at home to disarm, leading to sectarian tensions between Lebanese Shiite Hezbollah supporters and Sunni supporters from the opposing camp that have often spilled into deadly street fighting.

The civil war in Syria, the main transit point of weapons brought from Iran to Hezbollah, presents the group with its toughest challenge since its 1982 inception. Hezbollah’s support for the government of Syrian President Bashar Assad has proved costly and the group’s reputation has taken a severe beating at home. In addition, Assad’s problems could affect its main supply route from Syria; last week, Israeli warplanes bombed what was believed to be a shipment of sophisticated anti-aircraft missiles headed to Hezbollah.

The announcement also came ahead of a UN-backed tribunal for four Hezbollah members allegedly involved in the 2005 Beirut truck bombing that killed former Prime Minister Rafik Hariri, who was then Lebanon’s top Sunni politician. Hezbollah denies the charges and has refused to hand over the suspects.

New troubles for Hezbollah could also add to Iran’s international isolation. The Iranian regime is already under international sanctions for its suspect nuclear program, and has seen its position weaken due to its close ties with the Syrian regime. Its association with Hezbollah will likely further hurt Iran’s international image.

Wainwright warned the attack, along with a wave of attacks against Israelis around the world in the past year, is an indication of a real threat to Israelis and Jews in Europe.

“Even if it’s not Hezbollah, it has still obviously been carried out by an organization with some capability in the world, so the threat remains,” Wainwright said. “I don’t want to exaggerate the scale of that threat, but I think law enforcement authorities — government authorities — should take notice of this incident and prepare for the possibility at least of similar attacks in Europe.”