The US says it has concluded that Iran's president-elect was a leader of the group behind the 1979 hostage crisis at its embassy in Tehran.
Iranian veterans of the stand-off deny the president was involved
But it says it is unsure whether Mahmoud Ahmadinejad actively took part and is still looking into the matter.
Following Iran's Islamic revolution, radical students stormed the embassy and held 52 staff hostage for 444 days.
Former hostages say they recognise Mr Ahmadinejad, but Iranian veterans of the stand-off deny he was involved.
White House spokesman Scott McClellan told reporters: "In terms of the president, we've looked into the allegations that were made about his involvement in the 1979 hostage crisis.
"We know he was a leader of the student movement that organised the attack on the embassy and the taking of American hostages.
"However, we are still looking into whether or not he was actually one of the hostage-takers. That's something we continue to look into."
He added: "I don't think it's a surprise to anyone, given the nature of the regime in Iran, that he might have been involved in this kind of activity."
Mr Ahmadinejad beat moderate cleric Akbar Hashemi Rafsanjani in the second round run-off of last month's presidential vote. Washington said the elections were illegitimate.
The hostage crisis led to the breaking of diplomatic ties between Washington and Tehran, which have not been restored to this day.