Burgas, Bulgaria (CNN) -- Bulgarian police have been alerted to the possibility of a second suspect in a bomb attack that killed five Israeli tourists in Burgas, a high-level source in Bulgaria's Interior Ministry told CNN.
Investigators have determined that the bomb was detonated by a man caught on airport security videos about an hour before Wednesday's attack at Burgas International Airport. But they are now looking at the possibility that he had help.
According to the source, investigators have spoken to several eyewitnesses in attempt to identify the bomber.
They describe a man with short hair who spoke English with a "distinct" accent.
This is different from the man seen on the security video who has long hair, however, leading investigators to look into the possibility that a second person may be involved, a source said.
Police in Burgas have been given an identikit photo of the man and told to look out for anyone matching this description.
Earlier, a lower-level source at the Interior Ministry told CNN that police were looking for a second suspect, possibly an American.
That was later corrected by a higher-level source, who said a second suspect was a possibility, but had not been confirmed.
At least two hotels in Burgas told CNN that police had come with an identikit photo looking for a man in connection with the attack.
Five Israeli tourists and the Bulgarian bus driver died in the blast, as well as the bomber. More than 30 other Israelis were injured, some seriously.
Israel has said it suspects Iran or an Islamist militant group such as Hezbollah is behind the attack, a claim that has added to tension between Israel and Tehran. Iran, which has condemned the attack, has rejected Israel's claims.
The man who carried out the suicide bombing was carrying what the Interior Ministry has said was a fake Michigan driver's license.
Bulgarian Interior Minister Tsvetan Tsvetanov said Friday in a news conference broadcast on Bulgarian National TV that investigators were working round the clock to find new leads in the case, in collaboration with their Israeli counterparts and international organizations such as Interpol and Europol.
The suicide bomber was a foreigner who entered the country with fake identification documents and had been in Bulgaria for at least four days, the minister said. But authorities still don't know from where he came, where he stayed or what he did while in Bulgaria, he added.
The bomb is believed to have been concealed in a backpack placed by the bomber on a bus in a parking lot outside the airport.
Tsvetanov said Friday that he was not excluding the possibility that a second person might have been involved.
Experts are certain from DNA analysis of body parts found at the scene that the attack was carried out by a suicide bomber, he added.
The flag-draped coffins of the Israelis killed in the attack were flown in a military aircraft early Friday to Tel Aviv, where they were greeted with a ceremony. Thirty-six of those injured have also been transferred back to Israel for treatment.
The airport bus targeted by the attacker was to have taken 47 passengers to a beach resort.
Bulgarian officials have sought to reassure those worried by what is an unusual attack in the nation, saying security has been boosted wherever needed.