U.S. President Barack Obama has offered formal condolences and continued U.S. support to Bulgaria, as investigators probe the suicide bombing that killed five Israeli tourists and a bus driver Wednesday in the Black Sea port city of Burgas.

The official phone call to Prime Minister Boyko Borisov Friday came as Bulgarian police, the FBI and Interpol scoured the blast site and pressed to identify the bomber. Interior Minister Tsvetan Tsvetanov told reporters Friday the bomber was a foreigner who had been in the country for at least four days.

In Israel, where the five tourists were buried Friday, authorities continued to accuse the Iranian-backed militant group Hezbollah in the bombing. Iran has denied involvement.

Authorities describe the suspect, caught earlier on surveillance video, as a male about 36 years old who was carrying a bulky backpack shortly before the blast, as well as a fake driver's license.

Israeli officials say the Bulgaria bombing was part of a series of recent attacks targeting Israelis in Thailand, India, Georgia, Kenya and Cyprus.