The United States today challenged Iran to answer questions about the reported involvement of the nation's new president in the 1979 seizure of American hostages in Tehran after several former captives identified him as a ringleader.

Mahmoud Ahmadinejad, the hard-line mayor of Tehran who decisively won a presidential run-off election last week, was identified by at least half a dozen former hostages as one of the militants who took over the U.S. Embassy in November 1979 and held 52 Americans captive for 444 days. Several former hostages said Ahmadinejad was present during harsh interrogations.

However, former leaders of the takeover said in Iran today that Ahmadinejad was not part of their leadership group, and a spokesman for the president-elect denied that he played any role in the hostage-taking at all.

The controversy revived bitter memories of the embassy seizure and brought to the forefront a dispute that has remained unresolved for 26 years. Diplomatic relations between the two countries were broken over the hostage crisis and have never been restored.

President Bush, speaking to reporters about the upcoming G8 summit meeting in Scotland, said he has "no information" on Ahmadinejad's alleged role in the embassy takeover. "But obviously his involvement raises many questions," Bush said, adding that he was confident the answers would be found.

A State Department spokesman, Sean McCormack, said the government will "look into this question seriously" to establish the facts. "We have not forgotten" that U.S. diplomats were taken hostage and held for more than a year, he said. "The Iranian government, with respect to this question, has an obligation to speak definitively concerning these questions that have been raised in public by these stories."

The issue is "not a matter just for the United States," McCormack said. "What is at stake here are questions about the ability of diplomats around the world . . . to freely do their work while posted abroad."

For the Bush administration, Iran has presented one of the most challenging aspects of foreign policy. During his first term, divisions within the administration left the White House paralyzed over whether to contain Iran or more actively promote so-called "regime change." The language today reflected a tougher line the administration now intends to take toward the Iranian government, U.S. officials said.

Among the former hostages convinced of Ahmadinejad's role in their ordeal is retired Col. David M. Roeder, 66, who served as deputy Air Force attache at the embassy in Tehran. In a telephone interview from North Carolina, he said Ahmadinejad was present at about a third of his roughly 45 interrogation sessions following the embassy takeover.

Roeder, who was among the most harshly interrogated of the hostages, said Ahmadinejad "seemed to be the next level above the interrogators and the interpreters and the guards." Roeder said the reason he remembers him so vividly is that he was present the first time that interrogators made a specific threat to kidnap his handicapped son in the United States and mutilate him "if I didn't start to cooperate." Roeder said his captors knew his son's school bus number and the time he left home to go to special education classes, apparently as a result of surveillance in the United States.

"That was scary," he said.

Kevin Hermening, a Marine Guard at the embassy and the youngest of the hostages, said of the Iranian president-elect, "It sure seems to me that he was the person doing the interrogations that first day of the captivity. When I looked at his photograph during the election run-off, it looked like him."

Hermening, who ran for Congress twice as a Republican, added: "When I saw his photographs he looked like one of the guards. I do not remember him brandishing any weapons. But I do believe he was one of those leading the interrogations."

Other former hostages said in television interviews today that they also remembered Ahmadinejad as one of their captors after seeing video footage and photos of him from news coverage of the Iranian election.

After several former hostages came forward, federal agencies today began investigating, examining pictures of the hostage-takers and calling in former captives for interviews.

However, Ahmadinejad's office in Tehran vehemently denied the former hostages' assertions. Separate denials also came from two of the embassy takeover's top student organizers, both of whom are considered reformers opposed to Ahmadinejad's election.

"Even if 52 hostages say he was interrogating them, they're either lying or making it up," said Abbas Abdi, one of the former organizers, in a telephone interview from Tehran. "He didn't do it. He wasn't among them at all, for sure."

Iran's president-elect might have come through the U.S. Embassy compound as "many other students came in," Abdi said, speaking through an interpreter. "But he wasn't among the embassy students at all. He was definitely not a member of the students" who organized the seizure.

Ibrahim Asgharzadeh, who for years has acknowledged being the senior mastermind of the takeover, also said Ahmadinejad was not among the leaders.

"The leaders are all well known," he said. "Everybody knows them, and he was not among them."

Iranian officials noted that the three top leaders each represented a major university that belonged to the Office for Fostering Student Unity. Ahmadinejad attended the Science and Industry University, which was not among those universities and is located in a far eastern part of the sprawling Iranian capital.

Since the stunning election upset last week in which Ahmadinejad scored a landslide victory, the Bush administration has ratcheted up its criticism of the theocracy. The State Department Wednesday called for the "immediate and unconditional" release of Akbar Ganji, an Iranian journalist who has challenged Iran's leaders and advocated reform.

"Ganji's courageous efforts to investigate extra-judicial killings by Iranian security forces and his commitment to free speech and democratic government have earned him the respect of many around the world," a statement said.

The administration expressed concern about Ganji's lack of legal representation and his health, saying that "his mistreatment in prison is a serious violation of fundamental human rights."

This week the administration reissued a $5 million reward for three Lebanese who have been indicted for the 1985 hijacking of TWA flight 847 to Beirut. U.S. Navy diver Robert Dean Stethem was tortured and murdered during the ordeal and his body dumped on the Beirut airport tarmac.

One of the alleged lead hijackers is Imad Mugniyah, a member of Lebanon's Iranian-backed Hezbollah organization who has traveled frequently to Iran, according to U.S. officials.

Although the allegations against Ahmadinejad have stirred bad blood over the hostage crisis, some analysts pointed out that many of the former Iranian militants who participated in it have become reformers.

Among them is Asgharzadeh, a former city council member who urged students to "invite all the hostages to return to Tehran" as guests, not hostages, of the Islamic Republic. "Today we invite all the hostages to return to Tehran," he said at the 1998 commemoration of the takeover. "Today we have a new language for the world. We defend human rights."

Mohsen Mirdamadi, another top ringleader, became the chairman of Parliament's foreign relations committee in the late 1990s, at the height of the reform movement. He urged rapprochement with the West, including the United States.

Washington Post staff writer Karl Vick in Istanbul and special correspondent Mehrdad Mirdamadi in Tehran contributed to this report.