The officials are familiar with the investigation and spoke only on condition of anonymity because they are not authorized to speak to the news media.
U.S. officials expect Bulgarian investigators to link the militant group Hezbollah to the July 18 attack in the coastal city of Burgas, in which a bomb ripped through a bus that was carrying the tourists to their hotel. Bulgarian authorities are expected to release their findings on Tuesday.
Israel has claimed that Iran and Hezbollah played roles in the attack but investigators in Bulgaria have not made such a link previously, and Hezbollah has denied involvement in the attack.
U.S. officials said Monday that the White House and State Department were preparing statements with the expectation that investigators will make the link to Hezbollah. The officials spoke on condition of anonymity because they were not authorized to speak about the matter publicly.
One of the U.S. officials reiterated Washington's longstanding desire for the European Union to designate Hezbollah a terrorist organization. A public linking of Hezbollah to the Bulgaria bombing would increase pressure on the EU to do so.
Last month, the lead investigator in the case was dismissed after she told a Bulgarian newspaper that all three suspects were foreigners, with no local accomplices. The investigator, Stanelia Karadzhova, told Bulgaria's 24 Chasa daily that one of the suspects had been identified and that an arrest warrant had been issued.
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Karadzhova said new evidence suggested the bombing was not intended to be a suicide attack, as previously believed. Karadzhova said the bomber either pushed the detonator by mistake, or that somebody triggered the explosives remotely.
Officials had previously suggested Hezbollah could be involved in the attack. Pentagon press secretary George Little said shortly after the bombing that "the attack does bear some of the hallmarks of Hezbollah, but we are not in a position to make any final determination." Israel issued a complaint with the U.N. Security Council, accusing Iran.
Press; By MATTHEW LEE]
Rod McGuirk in Canberra, Australia, and Paisley Dodds in London contributed to this story.
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