A white male carrying a fake Michigan driver's license was identified as the likely suicide bomber who killed five Israeli tourists in the resort city of Burgas last week.
Bulgarian Prime Minister Boyko Borisov suggested the bomber likely had at least one accomplice. The operation was said to be sophisticated.
DNA and fingerprint evidence taken from the suspected bomber have turned up few leads and Borisov said following talks with U.S. counter-terrorism officials that it was too early to start speculating on possible backers.
"We can't cross the line and point a finger at who did this," he was quoted by The Wall Street Journal as saying.
Israeli officials accused Iran and Hezbollah of coordinating the attack. Iran denied the allegations.
The Journal notes that authorities in Cyprus this week arrested a man believed to have helped plan attacks on Israelis. The Israeli government said the 24-year-old suspect has ties to Hezbollah.