Bulgarian authorities found a fake US passport on the body of a man they believe was a suicide bomber who detonated himself on a bus full of Israeli tourists at the Burgas airport on Wednesday, the Sofia News agency reported on Thursday.

Initial reports suggested that the explosion was caused by a bomb placed in the luggage compartment of the bus. However, the Sofia News agency quoted Bulgarian Prime Minsiter Boyko Borisov as saying the attack was likely the work of suicide bomber.

Bulgarian security services were investigating the attack in conjunction with the FBI, CIA and Israeli authorities, according to the report.

The man described as a Caucasian with long hair and sports attire, was filmed walking around the airport for an hour prior to the attack by security cameras on the premises, according to the report. His body sustained the most damage in the blast, leading investigators to believe that he set off a bomb located on his person when he boarded the bus.

An FBI check of the document's found on the man's body, including the passport of a Michigan resident, determined that they were counterfeit, according to the report.

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