WASHINGTON — U.S. investigators have concluded that newly elected Iranian President Mahmoud Ahmadinejad is not the glowering Islamic militant seen escorting an American hostage in a 1979 photograph that was widely publicized this week, U.S. officials said yesterday.

The conclusion casts doubt on what had been considered a key piece of evidence that Iran's new president was among the leaders of the group of Islamic fundamentalists who seized control of the U.S. embassy in Tehran, the capital, and went on to hold 52 Americans hostage for 444 days.

A U.S. official familiar with the investigation of Ahmadinejad's role said that analysts had found "serious discrepancies" between the figure depicted in the 1979 photo and images of the Iranian president. The discrepancies included differences in facial structure and features, the official said.

"If there is a case to be made [that Ahmadinejad was among the hostage-takers in 1979]," the official said, "it doesn't look as if it will be done on the basis of those photographs."

The official stressed that the investigation is continuing and that it is still an "open question" whether Ahmadinejad was involved in the hostage crisis. Analysis of the photos was just one of many avenues in what has become a multi-agency inquiry, said the official, who spoke on condition of anonymity.

The Bush administration launched the investigation this week after several Americans who were among those taken hostage in Tehran 26 years ago said they recognized the new Iranian president as one of their captors.

Officials close to Ahmadinejad, as well as some Iranians who are known to have helped orchestrate the seizure of the American Embassy in 1979, have said Ahmadinejad was not involved.