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 seven 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.
Five tourists died along with the Bulgarian bus driver and the suspected bomber. Officials had said a sixth Israeli died overnight but this was later corrected.
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.
"All the signs lead to Iran," Israeli Prime Minister Benjamin Netanyahu said in a statement on Wednesday. "Israel will respond forcefully to Iranian terror."
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.
An Iranian foreign ministry spokesman said on Thursday that Tehran strongly condemned "all terrorist acts".
The Israeli foreign ministry said the bus had been carrying tourists from a charter flight that arrived from Israel.
Bulgarian officials on Thursday released CCTV footage of a man they suspect was the bomber. The man, who has very long hair and carries a backpack, is seen walking around the terminal and then leaving shortly before the blast happened.
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 Doskacharov, 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.
The suicide bomber is said to be 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".
Meanwhile, former Israeli national security adviser Uzi Arad confirmed on Thursday, for the first time, that Israel had assassinated Hezbollah chief Imad Mughniyeh in 2008.
Referring to the attack in Burgas, he told Israeli Army Radio: "Now is Iran's revenge."
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.