Is Iranian involvement a fact or an allegation?

However, Bassiouni said there was no evidence of Iranian involvement.

"But there's been tough fighting in the north," the diplomat said, "with increasing Iranian involvement."

"We see this attack as a very clear fingerprint of Iranian involvement," he said.

The issue of Iranian involvement in the bombing has been raised several times in recent weeks.

It was unclear who the killers were and whether the attack was tied to Iranian involvement in the negotiations.

The commission noted that most of the government's claims on Iranian involvement related to alleged intelligence operations, making them impossible to independently investigate.

While the evidence of Iranian involvement is strong, there are also signs that al Qaeda played some role, as yet unknown.

The police gave no details of the new evidence in the case, in which Iranian involvement has always been suspected.

"I must stress again that Iranian involvement at this point is strictly hypothetical," Mason said.