The information that led Mayor Michael R. Bloomberg to raise a public alert about a "specific threat" against New York City's subway system originated in Baghdad from an Iraqi informant who approached U.S. authorities in the past two weeks, U.S. officials said yesterday.

While officials in New York and Washington played down the credibility of the threat to varying degrees, the incident appears to be the first reported time that authorities in the United States have openly responded to information gathered in Iraq concerning an alleged domestic terrorist attack since the 2003 U.S. invasion.

According to a senior U.S. military officer, the report originated with an Iraqi informant who voluntarily approached U.S. authorities. Information was developed over the past two weeks, said a Bush administration official who spoke on the condition of anonymity because analysis is ongoing.

The military officer, who is in Iraq and has access to intelligence reports, said the story that the informant told was very detailed. It alleged a complex international scheme involving about 20 people in different countries, explosives packed in suitcases and a series of attacks targeting New York's transit network.

But a number of U.S. intelligence officials following the case have remained highly dubious about the story, the outlines of which bear close resemblance to the July 7 and 21 terrorist bombings in London.

"Frankly, the whole layout was so far-fetched that I looked at it skeptically," said the officer, who spoke on the condition of anonymity because of the sensitivity of the issue. "The story was fairly elaborate -- the description of the devices, the mix of personalities said to be involved. But then there were key things that the source claimed not to know that you'd think he would know if he knew all the rest."

The informant was given a polygraph test, which he passed, the officer said. A law enforcement official in New York said the source "had been proven before on al Qaeda operations."

U.S. and Iraqi forces proceeded this week to round up three people in Iraq who had been identified by the informant as planning to leave the country to participate in the alleged plot. The officer described the three as pharmacists, although the Bush administration official said only that at least one man was reported to be a pharmacist.

"The people turned out to be real and where the source said they would be," the military officer said. "But it's still not clear whether they have anything to do with any plot. We still basically only have one guy with this story."

The official in New York, who also requested anonymity because the investigation is ongoing, said a fourth individual -- in addition to the three men in Iraq -- may have traveled to New York and is being sought.

But the Bush administration official said, "There is no information to support that, nor is there information to suggest that individual even exists."

Details emerged as U.S. and New York City officials tried to minimize conflicting public statements about the seriousness of the threat. Bloomberg defended the deployment of hundreds of police onto subways, ferries and buses for what he called Thursday the most "specific threat to our subway system" that New York has ever received.

"If I'm going to make a mistake, you can rest assured it is on the side of being cautious," he said, according to the Associated Press.

Russ Knocke, a spokesman for Homeland Security Secretary Michael Chertoff, said Thursday that the threat was of "doubtful credibility" and the department was not raising the terrorism alert level nationally or in New York City. Yesterday, Knocke said that federal and city officials were "on the same page."

"I don't believe there was a disconnect," Knocke said. "You never get perfect intelligence, and in these situations there will always have to be a judgment call. We support his decision and respect his judgment."

On Thursday, FBI's assistant director for New York, Mark J. Mershon, said that actions taken had "partially disrupted this threat." But, he added moments later, "Thus far, there is nothing that has surfaced in that investigation, or enforcement actions, which has corroborated an actual threat to this city."

In New York, reinforced police units closed off a portion of Penn Station after a passenger on an Amtrak train reported a suspicious substance oozing out of a soda can. Investigators concluded the can contained sodium hydroxide and a green dye. "It appears as if it were a hoax," said police spokesman Paul J. Browne.

Researcher Julie Tate and staff writer Michelle Garcia contributed to this report.

A police officer stands guard outside the New York Stock Exchange after security was increased in the face of a threat to the subway system.