Bulgarian officials said Tuesday that the Lebanese group had been linked to the sophisticated bus bombing carried out by a terrorist cell that included Canadian and Australian citizens.
The announcement put pressure on European countries such as France and Germany, which haven't designated Hezbollah a terrorist organization despite the urgings of Israel and the U.S.
"If the evidence proves to be true, that Hezbollah is indeed responsible for this despicable attack, then consequences will have to follow," said Steffen Seibert, a spokesman for German Chancellor Angela Merkel.
He didn't specify what those consequences could be. But a ban on Hezbollah's activities in Germany, where authorities believe it has almost 1,000 members, could limit its ability to collect funds for the group's main branch in Lebanon.
Bulgarian Interior Minister Tsvetan Tsvetanov said Tuesday that two of the suspects in the July 2012 attack had been living in Lebanon for years.
"We have well-grounded reasons to suggest that the two were members of the militant wing of Hezbollah," he said.
Hezbollah, a Shiite militant group Lebanon that emerged in response to Israel's invasion of Lebanon in 1982, has been linked to attacks and kidnappings on Israeli and Jewish interests
But the group has denied involvement in the Bulgaria bombing and a German terrorism expert said the evidence against Hezbollah would have to be closely scrutinized.
Even if a link is established Germany would likely weigh the consequences of banning Hezbollah, which also acts as a political party and runs social services in Lebanon, said Guido Steinberg, a senior associate at the German Institute for International and Security affairs in Berlin.
"You need to be consistent when making terror lists and avoid political considerations," said Steinberg, who noted that in the past Germany has acted as a go-between for Israel and Hezbollah.
Adding Hezbollah to the European Union's list of terrorist organizations would require a unanimous decision by the foreign ministers of all 27 EU countries.