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.

New York City Police Officers check subway cars Friday at Columbus Circle. Security in the city's mass transit system has been increased after Thursday's announcement of a specific terrorist threat to the subway system.

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.

“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.