Mr. Asgari’s information has since been superseded by new disclosures, including that a second enrichment facility had been built in a mountain near the holy city of Qum. But even now, a critical negotiating issue is how many centrifuges Iran will be allowed to operate at these facilities.

On paper, Ali Reza Asgari would be a treasure trove for the C.I.A. He joined the Islamic Revolutionary Guards Corps soon after the 1979 revolution, and was sent to Lebanon in 1982, when Iran was bankrolling a wave of terrorism against Americans, through its proxy, the Islamic militant group Hezbollah. Later, he returned to Iran and rose to a senior post in the Revolutionary Guards, which oversees the nuclear program.

“He would have the crown jewels,” said Robert Baer, a retired C.I.A. agent who had his own career in the Middle East and spoke to Mr. Bird for his book.

But while Mr. Baer said Mr. Bird’s reporting is persuasive — he said he knows some of the sources the author consulted in the region — he noted that the book contains no smoking gun establishing Mr. Asgari’s whereabouts. Indeed, Mr. Asgari may no longer be in the United States.

Mr. Bird said that when he asked a former senior Bush official about the decision to grant Mr. Asgari asylum, he received a cryptic reply: “At the unclassified level, I cannot elaborate on this issue.” He cited a report in Der Spiegel, the German newsmagazine, that Mr. Asgari twice called a fellow Iranian defector — from Washington, where he had been held in a C.I.A. safe house, and from “somewhere in Texas.”

Stuart H. Newberger, a Washington lawyer who represents victims of the 1983 attack, said he believed the book was accurate, though he could not corroborate the Asgari disclosure himself. He said he had supplied Mr. Bird with trial transcripts and internal government documents he had obtained for his litigation.

“Asgari got a get-out-of-jail-free card because of the Iran nuclear issue,” Mr. Newberger said.

For the Obama administration, Mr. Bird’s revelations could be awkward. Mr. Newberger said it should make terrorism an issue in any negotiation about relaxing sanctions against Iran. But the White House has tried to keep the nuclear negotiations tightly focused on technical questions of Iran’s enrichment capability and international inspections.