New York Mayor Michael Bloomberg defended his decision Friday to go public with a terror threat to the city's subway system.

The mayor ordered stepped-up police patrols after an informant apparently warned of a plan to bomb the subway -- perhaps with bombs hidden in baby strollers and briefcases.

However, officials from the Department of Homeland Security have suggested the intelligence was of dubious credibility.  A former senior FBI official told CTV News that security officials in Washington did not think there was a credible threat and questioned the mayor's decision to warn the public.

"If I'm going to make a mistake, you can rest assured it is on the side of being cautious," Bloomberg said in response to the criticism.

U.S. President Bush refused to criticize the mayor's decision.

"I think they took the information that we gave and made the judgments that they thought were necessary," he said.

Evacuations in N.Y. and Washington

Morning commuters in the city were barred for more than an hour from entering the area where Amtrak commuter trains arrive and depart beneath Madison Square Garden.

After the station was reopened around noon, Amtrak officials said a discarded soft drink bottle -- filled with an unidentified green liquid -- found during the morning rush had prompted the closure.

New York City Police Commissioner Ray Kelly told an afternoon press conference that the security scare was a false alarm.

A bomb threat prompted the evacuation of the Washington Monument in the U.S. capital Friday afternoon.

The Washington threat was called in to local police at about 2:30 p.m. ET. Though bomb-sniffing dogs moved in and two blocks around the monument were closed off, nothing was found.

Arrest in Iraq

The entire New York City subway system has been on alert since Thursday, when officials warned of a specific threat of an attack on transit network.

Travellers were asked to report any suspicious people or activities, as police searched bags, briefcases and even baby strollers across the sprawling transit system.

U.S. forces in Iraq arrested two alleged plotters who had been under close surveillance on Thursday, while a third suspect was apparently arrested Friday.  A fourth suspect had reportedly already travelled to New York.

A law enforcement official familiar with the case told The Associated Press the man's trip to New York was described by an informant who had spent time in Afghanistan and proved reliable in past investigations.

"He's been a source of multiple correct information in the past,'' the official told AP, speaking on condition of anonymity because of the continuing investigation. "Does that mean a fourth person he identified is in fact in New York? We don't know that."