The alleged threat that led to heightened security on New York subways last week may have been a hoax on the part of an Iraqi informant attempting to get money in exchange for information, U.S. intelligence and counterterrorism officials said yesterday.

The informant has since disappeared in Iraq, and the Defense Department has not been able to locate him, city and federal officials said.

New York Mayor Michael R. Bloomberg described the informant's claims last week as the "most specific threat" ever received against the city's transit system, leading officials to issue a heightened terrorist alert and blanket the subways with police and National Guard troops.

U.S. troops in Iraq captured three suspects south of Baghdad who the informant said were involved in the alleged plot.

But none of the suspects, including two who were given polygraph examinations, corroborated the informant's allegations or appeared to have any connection to a terrorist plot, according to intelligence officials.

The city lifted the alert Monday after the time period identified by the informant passed without incident.

Officials with the FBI and the Department of Homeland Security were highly skeptical of the threat from the beginning, though federal officials sought to play down any differences with New York authorities.

The informant, who approached U.S. authorities voluntarily in Baghdad in the past two weeks, detailed an alleged plot by about 20 international conspirators to attack the New York transit system over the weekend with bomb-laden suitcases, baby strollers and other items.

New York Police Commissioner Raymond W. Kelly told reporters that the source of the threat information is not in U.S. custody. A military officer following the case said that the Iraqi informant has broken off communications with American intelligence agents.

"We don't seem to have contact with him at the present time," the officer said. "He appears to be in hiding or on the run."

Department of Homeland Security spokesman Russ Knocke, who called the threat "noncredible" last week, declined to elaborate yesterday.

"The intelligence community has not found any evidence to substantiate the threat information," Knocke said. The FBI also declined to comment in detail.

Bloomberg, who is running for reelection in November, defended the city's decision to ratchet up security. At a news conference yesterday, he said officials had little choice but to respond to such a detailed claim.

"This was a planned attack that had a specific time and target and method," Bloomberg said. "It was the first really serious allegation of a direct attack on this city" since the Sept. 11, 2001, attacks, he said.

Several U.S. intelligence officials said that once it became clear that none of the informant's claims about New York could be corroborated, authorities began exploring the possibility that he had invented the story to curry favor or receive financial rewards.

Some counterterrorism experts noted last week that elements of the alleged plot bore similarities to the July 7 subway bombings in London and the attempted July 21 attack there.

A U.S. government official also said that at least one Latin American militant group has been blamed for using a baby stroller in a terrorist attack and similar plans have been described in works of fiction.

Staff writer Bradley Graham

and researcher Karl Evanzz contributed to this report.

Soldiers at Penn Station are among precautions against an attack on New York's transit system.