Bulgarian police, the FBI and Interpol are struggling to identify a suicide bomber who killed six people, including five Israelis, as the US said the attack bore the "hallmarks" of Hezbollah.
Investigators have released CCTV footage of the person they believe carried out Wednesday's attack in the Black Sea airport of Burgas on a bus carrying Israeli holidaymakers, which also claimed the life of the Bulgarian driver.
After two days of investigations, Interior Minister Tsvetan Tsvetanov was only able to confirm Friday that the bomber was "not a Bulgarian citizen" and had been in the country "not less than four days".
Fingerprints and DNA samples from the suspected bomber's body were being used in the effort to learn his identity, he added.
The minister also made public Friday evening some scanty details on the explosive used in the first attack of its kind in Bulgaria.
"We are speaking about trotile, about three kilogrammes. Analyses are continuing," he told state BNT television.
Trotile is made from Trinitrotoluene or TNT -- a common material in military bombs and also frequently used in attacks by militant groups.
Israel and Bulgaria meanwhile held emotional funerals for the victims -- Israelis Kochava Shriki, 44, who was pregnant, Yitzhik Kolengi, Amir Menashe, Elior Priess and Maor Harush, all in their 20s, and the Bulgarian driver of the bus Mustafa Kyosov, 36.
More than 30 people were also injured in the blast, three of them seriously, all of whom have since been repatriated, as have around 60 others who escaped injury. Some opted to continue their holiday in the Black Sea resorts popular with Israelis.
US President Barack Obama pledged solidarity with Bulgaria over the "barbaric attack," echoing indignation from around the world.
In a telephone call with Bulgarian Prime Minister Boyko Borisov, Obama "offered his support for the ongoing investigation, and for the Bulgarian people in this challenging time," the White House said in a statement.
On Wednesday, Obama had phoned Israeli Prime Minister Benjamin Netanyahu to offer his condolences and support.
Israel has blamed Iran and Tehran's "terrorist proxy" Hezbollah, saying it fitted a pattern of other recent attacks or attempted attacks on Israelis including in Thailand, India, Georgia, Kenya and Cyprus.
The Islamic Republic, already under pressure over growing international tensions over its nuclear programme, rejected Israel's accusations as "ridiculous".
"The attack does bear some of the hallmarks of Hezbollah but we're not in a position to make any final determination on who was responsible," Pentagon press secretary George Little told reporters on Friday.
Airport video footage of the presumed suicide bomber released by Bulgarian authorities showed an apparently white male with long hair, possibly a wig, dressed in typical holiday gear -- shorts, a baseball cap, sneakers -- and carrying a backpack and a laptop bag.
Tsvetanov said the man, who was shown wandering around the airport, looked around 36. Investigators have also recovered his driving licence from the US state of Michigan that is thought to be fake.
The name on the licence, an image of which was shown on Bulgarian television, is Jacque Felipe Martin of Baton Rouge, Louisiana, born in 1987. Michigan is known to only issue licences to state residents.
Bulgarian prosecutor Kalina Chapkanova told private bTV television Friday that the suspect had used the fake licence when he tried to rent a car in the nearby town of Pomorie the day before the attack.
The car rental owner turned him down as the document seemed suspicious.
She also cited witnesses as saying that he spoke English with an accent, possibly an Arab one.
The owners of the car rental agency confirmed that the man who tried to hire a vehicle had very short hair, dark skin and "looked like an Arab." He also did not resemble the photo on the licence, which showed someone with long hair.
Interpol has sent an "incident response team" to Bulgaria including two "terrorism" experts from Switzerland and France and a US explosives specialist. European police agency Europol was also assisting.
The explosion ripped through the bus as around 50 Israeli tourists arriving from Tel Aviv were loading their bags before travelling to nearby holiday and gambling resort Sunny Beach.
The bomber struck on the 18th anniversary of an attack on a Jewish community centre in Argentina that killed 85 people.