The bombing of an Israeli tourist bus in eastern Bulgaria was probably carried out by a male suicide bomber with fake US documents, officials say.
At least eight people died and 34 were injured when the bus exploded at Burgas airport, by the Black Sea.
Israel has sent planes to Burgas with doctors and officials to bring back the dead and injured.
Israeli Defence Minister Ehud Barak said Lebanese Hezbollah was the direct perpetrator, under Iran's auspices.
The BBC's Jon Donnison, in Jerusalem, says the attack could be part of a covert but violent war between Israel and Iran, and there is a view among some analysts that this attack could be a response to a series of recent attacks on Iranian nuclear scientists.
Mr Barak said the attack resembled several recent attempted attacks on Israeli targets in India, Thailand, Azerbaijan, Kenya and Cyprus.
But he said that he did not think there had been any specific prior information that the attack was imminent.
The Israeli foreign ministry said the bus was carrying tourists from a charter flight that arrived from Israel.
The BBC's Chris Morris, in Sofia, said a Bulgarian official has told the BBC that CCTV footage from the airport shows a Caucasian male hanging about for some time in the airport terminal building before the explosion took place. The man is seen leaving the terminal shortly before the blast happened just outside.
Investigators are working on the theory that this man is the bomber, the official said.
Israeli officials said passengers from a Tel Aviv-Burgas flight boarded the bus shortly after 17:00 local time (14:00 GMT) on Wednesday.
"I was on the bus and we had just sat down when after a few seconds we heard a really loud explosion," Gal Malka told Israel's army radio.
"The whole bus went up in flames," she said, adding that the explosion took place near the front of the bus.
Bulgarian journalist Dobromir Dovkacharov, who arrived at the scene about 30 minutes after the blast, said: "I saw three buses completely burnt out - just the metal bars were left.
"There were crowds of people around, very distressed. One man said he saw decapitated heads. Others spoke of body parts flying through the air," Mr Dovkacharov said.
Six tourists died, along with two others, one of them the Bulgarian bus driver.
The eighth fatality is now thought to be the suicide bomber, a man with a Michigan-issued US driver's licence which reports said may have been fake.
Two of the wounded are in a serious condition and have been flown to the capital Sofia.
Israeli forensic teams who arrived during the night are helping to identify the bodies.
Wednesday's blast came on the 18th anniversary of a deadly attack on a Jewish community centre in Argentina. Israel blamed Iran for that attack - a claim denied by Tehran.
US President Barack Obama described the bombing as a "barbaric terrorist attack".
Bulgaria is a popular tourist destination for Israelis.
However, in January there were reports that Israel had asked Bulgaria to tighten security for Israeli tourists travelling by bus.
This followed a reported discovery of a suspicious package found on a bus with Israeli tourists travelling from Turkey to Bulgaria.