The Iranian nuclear scientist who claims to have been abducted by the CIA was paid more than $5 million to provide intelligence on Iran's nuclear program, according to a published report.

The scientist, Shahram Amiri, returned to Iran early Thursday where he was given a hero's welcome after claiming he was abducted by U.S. agents and then offered $50 million to stay in the U.S.

The Washington Post reported in its online edition late Wednesday that Amiri worked for the CIA for more than a year before abruptly asking to return home this week. The Post said he was paid $5 million out of a secret program aimed at inducing scientists and others with information on Iran's nuclear program to defect.

The U.S. says he was a willing defector who changed his mind and decided to board a plane home from Washington.

Amiri is not obligated to return the money, the newspaper reported, however he may be unable to access the funds after breaking off "significant cooperation" with the U.S.

"Anything he got is now beyond his reach, thanks to the financial sanctions on Iran," a U.S. official told the Post. "He's gone, but his money's not. We have his information, and the Iranians have him."

The payment of millions of dollars to Amiri appears to bolster the U.S. government's assertion that he was neither abducted nor brought to the country against his will, the Post said. The size of the payment could be a gauge of how valuable the information gleaned from him was to the CIA.

Amiri claimed Thursday he suffered extreme mental and physical torture at the hands of U.S. interrogators after disappearing last year, adding to Tehran's allegations he was abducted by American agents.

Amiri was embraced by his family -- including his tearful 7-year-old son -- after arriving in Tehran in the latest spectacle of a puzzling series of events that left Iran and Washington with starkly different accounts.

Iran has portrayed the return of Amiri as a blow to American intelligence services that were desperate for information on Iran's nuclear program. Iran has sought maximum propaganda value -- allowing journalists to cover Amiri's return and having a top envoy from Iran's Foreign Ministry on hand to greet him.

Washington described the 32-year-old Amiri as someone who reached out to U.S. officials, but have offered few other details.

Speaking to journalists after a flight via Qatar, Amiri repeated his earlier claims that he was snatched while on a pilgrimage last year in the Saudi holy city of Medina and carried off to the United States.

He claimed he was under intense pressures during the first few months after his alleged kidnapping.

"I was under the harshest mental and physical torture," he said at Tehran's international airport, with his young son sitting on his lap.

He also alleged that Israeli agents were present during the interrogations and that CIA agents offered him $50 million to remain in America. He gave no further details to back up the claims or shed any new light on his time in the United States, but promised to reveal more later.

"I have some documents proving that I've not been free in the United States and have always been under the control of armed agents of U.S. intelligence services," he told reporters.

He also sought to downplay his role in Iran's nuclear program -- which Washington and allies fear could be used to create atomic weapons. Iran says it only seeks energy-producing reactors.

"I am a simple researcher who was working in the university," he said. "I'm not involved in any confidential jobs. I had no classified information."

His case was often raised by Iranian officials in the past year, but Washington offered no response. It took a higher profile after Iranian authorities decided to pursue charges against three Americans captured in July 2009 on the Iran-Iraq border.

Last month, Iranian state TV aired a video he purportedly made from an Internet cafe in Tucson, Arizona, and sent to Iranian intelligence claiming U.S. and Saudi "terror and kidnap teams" snatched him. In another, professionally produced one, he said he was happily studying for a doctorate in the United States. In a third, shaky piece of video, Amiri claimed to have escaped from U.S. agents in Virginia and insisted the second video was "a complete lie" that the Americans put out.

U.S. officials never acknowledged he was on American soil until Tuesday, hours after he turned up at the Iranian interests section at the Pakistani Embassy in Washington, asking to be sent home. Secretary of State Hillary Rodham Clinton said Amiri had been in the United States "of his own free will and he is free to go."

In an interview with Iranian state Press TV from the interests section before heading home Wednesday, Amiri elaborated on his abduction account and denied he was ever a willing defector.

"If I had sought asylum (in the U.S.), why did I not take my family out (of Iran)? What was the reason for me to escape Iran and seek asylum without sending my family out first?" he said in the interview, aired Wednesday.

He said he was in Medina when three men in a van posing as fellow pilgrims offered him a ride. "As I sat down, the man in back held a gun toward me and told me to keep quiet," he said. "They took me to a secret place and injected me, and when I woke up I saw myself in a huge airplane" and was taken to America.

There, CIA agents "pressured me to help with their propaganda against Iran," he said, including offering him up to $10 million to talk to U.S. media and claim to have documents on a laptop against Iran.

"I promised myself that I wouldn't talk against my country at all," Amiri told Press TV. Instead, he said, he tried to string the CIA along, letting them settle him in Tucson, where he suggested he had relative freedom there on the condition "I not talk about my abduction or what happened afterward."

But after they discovered he had made the first video, in April, "they relocated me from Tucson to Virginia with guards all around me and until this moment, I've been monitored by armed agents."

"They put more psychological pressure on me. They told me they would kill me ... . They threatened me every time," Amiri said.

U.S. officials would say little about the circumstances of Amiri's defection and what went wrong. But there were suggestions that threats to his family in Iran pushed Amiri to first make the claims he was kidnapped.

Amiri had originally "left his family behind, that was his choice," said a U.S. official who was briefed on the case, speaking on condition of anonymity because the official was not authorized to talk publicly about the case.

Vincent Cannistraro, a retired CIA officer, said he believes Amiri was not recruited by the CIA but volunteered to provide information to the agency about Iran's nuclear program over a period of years before he came to the U.S. Cannistraro said he believed that after Amiri's defection, the Iranian government threatened to harm his son as leverage to get him back to Tehran.

"It certainly was an embarrassment to the Iranian government, and clearly they wanted him back," Cannistraro said.

There are relatively few known U.S. cases of defectors changing their minds and returning to their homeland.

Vitaly Yurchenko, head of North American espionage for the Soviet Union's KGB, defected in 1985, only to "redefect" a few months later, claiming he was not a traitor but rather a kidnap and drugging victim. He returned home and was awarded a medal.

Two sons-in-law of former Iraqi leader Saddam Hussein met a far worse fate. Hussein Kamel al-Majid and his brother, Saddam Kamel, defected from Iraq to Jordan in August 1995, along with their wives -- Saddam's daughters. When the brothers returned to Iraq they were killed, reportedly by relatives in Baghdad angered by their betrayal.

Before he disappeared, Amiri worked at Tehran's Malek Ashtar University, an institution closely connected to the country's powerful Revolutionary Guard.

State Department spokesman P.J. Crowley said he does not know what Amiri may have told U.S. officials, but that the U.S. government "maintained contact with him" during his stay in the United States. Pressed whether Amiri was a defector, Crowley replied, "I just don't know the answer."

The Associated Press contributed to this report.