NEW YORK - The investigation into an alleged plot to bomb the city's subway moved forward on several fronts Friday as a third suspect was arrested in Iraq and authorities looked into whether a fourth person had traveled to New York as part of the scheme, officials said.
A law enforcement official familiar with the case said 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 said, 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."
The official added that authorities had not confirmed whether the fourth man even exists.
Alarmed by the informant's report of a plot to attack city subways with as many as 19 bombs in bags and possibly baby strollers, U.S. forces in Iraq arrested two suspected plotters who had been under close surveillance until Thursday morning, officials said. The third escaped until his arrest Friday.
City officials posted thousands of additional uniformed and plainclothes officers throughout the subway system and warned New Yorkers to keep their eyes open for anything out of the ordinary.
The announcement sparked behind-the-scenes jostling with security officials in Washington, who downplayed the threat and suggested that Mayor Michael Bloomberg may have overreacted.
Bloomberg and Police Commissioner Ray Kelly vigorously defended their reaction Friday.
"We did exactly the right thing," Kelly said.
Those arrested had received explosives training in Afghanistan, a law enforcement official said Friday. They had planned to travel through Syria to New York, and then meet with an unspecified number of operatives to carry out the bombings. The official said that the threat was "specific to place," and that the window for the attack was anywhere from Friday through at least the weekend.
A federal official said one of the suspects arrested in Iraq apparently told interrogators that more than a dozen people were involved in the plot, and that they were of various nationalities, including Afghans, Syrians and Iraqis, the official said.
"There could be one or many," the official said, who had been briefed on the case and spoke on condition of anonymity because the investigation is continuing. "We just don't know. There may not be any."
Kelly, Bloomberg and other city officials declined to release details of the alleged plot, saying much of the information was classified.
Bloomberg called the plot report the most specific terrorist threat that New York officials had received to date and said it was essential that authorities err on the side of caution when protecting the city of 8 million.
"If I'm going to make a mistake you can rest assured it is on the side of being cautious," Bloomberg said at a news conference, flanked by Kelly.
A law enforcement official said the informant had failed parts of a polygraph test but appeared to be telling the truth in response to questions dealing with the alleged plot.
But Homeland Security officials in Washington said the threat was of "doubtful credibility."
President Bush said Friday that New York City officials exercised their prerogative in publicizing the threat. Asked if he thought New York officials had overreacted, Bush said: "I think they took the information we gave and made the judgments they thought were necessary."
Near the Port Authority Bus Terminal on Friday, more officers were visible on the streets, and one lane of traffic on Ninth Avenue was reserved for emergency vehicles.
"Hopefully, God's with me and I'll be OK," Vinnie Stella said as he entered the subway at Penn Station.
Rob Johnson, 30, said he was not worried: "The cops have it under control."
In Baghdad, spokesmen for the U.S. military and the U.S. Embassy declined to comment on the arrests.
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 in the wake of the train bombings in London.