The Pentagon says an attack this week on Israeli tourists in Bulgaria "bears some of the hallmarks of Hizballah."
Pentagon spokesman George Little made the comment after Israeli Prime Minister Benjamin Netanyahu earlier blamed Iran and the Lebanese militant group for the July 18 suicide bombing, which killed five Israeli tourists.
Tehran and Hizballah deny any connection.
The attack at Burgas airport also killed the Bulgarian bus driver and the bomber and injured more than 30.
Bulgarian authorities on July 20 said the suicide bomber was a foreign citizen.
Also on July 20, the five victims, who were returned to Israel by military transport overnight, were buried.
U.S. President Barack Obama called Bulgarian Prime Minister Boyko Borissov, and condemned "the barbaric attack."
The FBI is assisting Bulgarian authorities in the investigation.