The man who blew himself up on a bus full of Israeli tourists, killing himself and six other people had tried to rent a car but was turned down because his ID appeared suspicious, a Bulgarian prosecutor said.
Authorities are looking for clues as to who the suspect was, using his fingerprints, his DNA and his fake Michigan driver's licence.
Prosecutor Kalina Chapkanova said in a TV interview that, prior to the attack, the man tried to rent a car in the town of Pomorie, near the site of the bombing.
She said the owner of the rental agency became suspicious of his licence and refused to conclude the deal.
Ms Chapkanova quoted the agency owner as saying that the suspect spoke English with a "specific" accent but she refused to give details.
Israel was quick to blame Iran and its Lebanese ally Hezbollah for Wednesday's attack at Bulgaria's Burgas airport and a US official said Hezbollah was believed to be behind the attack.
The victims included the Bulgarian bus driver and five Israelis, including a pregnant woman.