“We’re deferring to our Bulgarian partners to make these determinations and to announce at the appropriate time what the findings in their investigations are,” he said, adding that “there are clear indications that Hezbollah and Iran have been involved in terrorist plotting against innocents in many parts of the world.”

Iran has denied any involvement in the bombing. Bulgarian, Israeli and American officials have cooperated closely in the aftermath of the attack. “We will continue to help Bulgaria in any way we can in this investigation,” Mr. Brennan said. He read from a letter Mr. Obama wrote to Mr. Borisov, calling the attack “a terrible reminder of the danger we face and the challenges that we must confront as allies.”

The latest revelations appeared not only to rule out the possibility of a lone attacker but also to point toward an organized terrorist group. Over the weekend one Lebanese newspaper received an unsubstantiated claim of responsibility supposedly from Al Qaeda. But terrorism experts have said the attack fit the pattern of the continuing covert actions by Iran and its proxy, Hezbollah. Investigators were still trying to reconstruct the movements of the suspects and the whereabouts of the accomplices, who did not die in the attacks.

The Bulgarian police have canvassed hotel managers up and down the Black Sea coast, questioned taxi drivers by the hundreds and, wherever possible, taken any available footage from security cameras that might have caught the suspects on tape. Mr. Borisov described a sophisticated group behind the bombing.

Last week the Bulgarian authorities released the video footage of the man that they said was the suspect, who was wearing a large backpack in the video and found to be carrying a fake Michigan driver’s license after the attack. Over the weekend, Galina Mileva, a former forensic doctor who is now a member of Parliament from the governing party, told a reporter that the bomber had blue eyes and fair skin.

Ms. Mileva, who arrived in the autopsy room toward the end of the examination, said that the bomber appeared to be wearing the backpack when it detonated. “From my experience, it appears that the most likely location of the explosion was on the back because the torso was so torn up,” she said.