Israeli Prime Minister Benjamin Netanyahu blamed the attack on Hezbollah, an Iranian-backed Lebanese Shiite Muslim guerrilla group.
The attack happened at a Black Sea resort town in Bulgaria on Wednesday. A total of seven people - five Israelis, the Bulgarian driver and the bomber - were killed and dozens more were injured.
The suspected bomber was seen on an airport security camera shortly before the bombing. He was seen carrying a large backpack with wheels. Officials said the bomb detonated in the luggage compartment of the bus as the attacker mingled among his victims.
Authorities are still trying to determine his true identity, but Interior Minister Tsvetan Tsvetanov said the bomber was believed to have been about 36 years old and had been in the country between four and seven days.
According to Bulgarian Prime Minister Boiko Borisov, U.S. officials reported having no such person in their database matching the identity on the Michigan driver's license. Michigan is home to one of the largest Arab communities in the U.S.
Although no group claimed responsibility for the attack, Israel has attributed a series of attacks on its citizens in recent months to Iran and its Shiite proxies, threatening to escalate a shadow war between the two enemies. Iran has denied any involvement in the attack.
The Associated Press contributed to this report.