A bombing that killed eight people and injured dozens on a bus full of Israeli tourists was probably a suicide attack, according to Bulgaria's interior minister.
Tsvetan Tsvetanov said the suspected attacker was carrying a Michigan driver's licence, which has now been sent to the FBI for authentication.
The death toll has also risen to eight after the Bulgarian bus driver died in hospital.
Six of the victims are Israeli citizens, while the nationality of the suspected bomber remains unknown. Some 30 others were wounded in the attack.
The Israelis had just arrived in the Bulgarian Black Sea resort city of Burgas, on a charter flight from Tel Aviv carrying 154 people, including eight children.
Passengers said they were boarding the bus in the airport car park when the blast ripped through the white vehicle.
"We were at the entrance of the bus and in a few seconds we heard a huge boom," said Gal Malka, an Israeli teenager who was slightly wounded.
No group has yet claimed responsibility, but suspicion has fallen upon Iran and the Hizbollah guerrilla group.
"All signs point to Iran. Just in the past few months, we have seen attempts by Iran to harm Israelis in Thailand, India, Georgia, Kenya, Cyprus and more,” said Israel's Prime Minister Benjamin Netanyahu.
"This is an Iranian terror attack that is spreading across the world. Israel will react forcefully to Iran's terror."
The resort is some 250 miles (400km) east of the capital, Sofia. It has become a popular travel destination in recent years for Israelis, particularly for recent high school graduates before they are drafted for mandatory military service.
Despite repeated alerts and concerns of an Iranian-backed attack in recent months, Israel said it had no advance intelligence of a pending attack in Bulgaria.
In Sofia, mayor Yordanka Fandakova has ordered a stronger police presence at all public places linked to the Jewish community. There are some 5,000 Jews in Bulgaria, and most live in the capital.
A Bulgarian government plane will fly about 100 Israelis who were not wounded back to Israel.