Authorities stepped up security Thursday after receiving what city officials called a credible threat that the New York subway (search) could be the target of a terrorist attack in coming days. But Homeland Security officials in Washington downplayed the threat, saying it was of "doubtful credibility."

The threat involved the possibility terrorists would pack a baby stroller with explosives, among other potential subway bombing methods, a law enforcement official said, speaking on condition of anonymity because the investigation is ongoing.

New York officials responded by mobilizing police officers to begin looking through commuters' strollers, bags, brief cases, and luggage.

"This is the first time we have had a threat with this level of specificity," Mayor Michael Bloomberg (search) said at a nationally televised news conference alongside Police Commissioner Raymond Kelly, adding that he still felt secure enough to take the subway home Thursday night.

But in Washington, Homeland Security Department spokesman Russ Knocke said "the intelligence community has concluded this information to be of doubtful credibility. We shared this information early on with state and local authorities in New York." Knocke did not elaborate.

A counterterror official, who was briefed about the threat by Homeland Security authorities, said the intelligence was considered doubtful because it did not reflect "on-the-ground, detailed" information. Rather, the official, who also insisted on anonymity, said the intelligence was similar to "what can be found on the Internet and a map of New York City."

The law enforcement official in New York said that city officials had known about the threat at least since Monday, but held the information until two or three Al Qaeda (search) operatives were arrested in Iraq within the past 24 hours. Once the arrests were made, officials felt they could go public, the official said.

Authorities are concerned, the official said, that there might be Al Qaeda operatives in New York City connected to the plot. They have no hard evidence of that, but are investigating.

The U.S. military spokesman's office in Baghdad had no information on the operatives' arrests. Pentagon spokesman Bryan Whitman said he had seen no indication of a U.S. military operation to round up Al Qaeda suspects.

The law enforcement official said the threat was "specific to place," and that the window for the attack was anywhere from Friday through at least the weekend.

Bloomberg called it the most specific terror threat New York officials had received to date. No one in the city has been arrested or detained, he said.

"We have done and will continue to do everything we can to protect this city," he said. "We will spare no resource, we will spare no expense."

Gov. George Pataki said the state would call up hundreds of National Guard troops while seeking help from law officers in Connecticut and New Jersey to patrol commuter trains.

Some commuters took the threat in stride.

Paul Radtke, 45, of Hoboken, N.J., said he has heard similar warnings before and found it hard to take them all seriously.

"Unless it's something dramatic that's happening, I've got to go to work," Radtke said after getting off a subway train at Penn Station. He said the only travel habit he is changing is trying not to make eye contact with police officers so they won't search his bag.

An estimated 4.5 million passengers ride the New York subway on an average weekday. The system has more than 468 subway stations. In July, the city began random subway searches following the London train bombings.

Leila Fullerton was about to board a subway for Brooklyn after leaving work, where she had gotten word of the threat. "I'll think about it, but I'm not scared, really," she said.

But she added that since the London bombings, she has felt nervous at times and found herself scanning the subway car at times looking for suspicious characters.

"It's a terrible feeling going down there sometimes," she said, gesturing at the subway stairwell.

New York City has been on high alert — or code orange — on the nation's terror threat advisory system since the Sept. 11, 2001, terror attacks. There are no plans to raise its alert level in wake of the threat, Knocke said, nor are authorities considering changing the nationwide elevated threat level, or code yellow.

Bloomberg said there was no indication the threat was linked to this month's Jewish holidays.