Police in Bulgaria were investigating Sunday whether there may have been two more suspects involved in the bombing of a bus last week which killed five Israeli tourists and their Bulgarian driver, local media reported.

The Bulgarian daily Standard Sunday quoted police sources as saying that the two suspects were to backup and control the suicide bomber at Burgas airport, where Wednesday's blast took place.

The first was supposed to stand in if the primary attacker was foiled, while the second was supposed to kill the first two in case they backed out or were arrested before the attack, novinite.com reported.

Israel has blamed Iran and Hezbollah for the attack, the latest in a series on Israeli tourists and diplomats abroad this year.

Bulgarian authorities are being helped with investigators from the US and Israel over the attack, which targets passengers arriving off a flight from Tel Aviv for a holiday on the Black Sea coast.

On Friday, Bulgarian Interior Minister Tsvetan Tsvetanov said that the identity of the suicide bomber was still unclear, despite media reports on the matter.

DNA samples taken from the terrorist's body were sent to U.S. intelligence agencies, but, as yet, no match has been found with any former Guantanamo detainees. Information was also sent to Interpol and was distributed to police forces around the world, however his identity was not revealed.

Another attempt made was to link the explosives used in the attack to previous bombings. If successful, it may point to the people responsible for the attack.