There are reports the terrorist threat that spurred a security clampdown on the New York City subway system was a hoax.

On Tuesday, CNN reported that an informant in Iraq admitted information about an al Qaeda plot to attack New York commuters was false and intentionally misleading.

Officials said thousands of additional police officers scouring trains and stations were needed to thwart a suspected plot to use baby strollers and briefcases packed with remote-controlled explosives.

The city's subway system was put on alert for four days starting Friday in a bid to stem any such attack.

The informant, who had spent time in Afghanistan, first told U.S. intelligence that a group of men were plotting to attack New York subways with timed or remotely detonated bombs.

After U.S. forces in Iraq arrested two of the alleged plotters Thursday, New York City Mayor Michael Bloomberg, Kelly and the FBI's New York office announced the increased subway security. A third suspect was arrested Friday.

But officials have since conceded the interrogation of the initial informant and a pair of suspected co-conspirators detained by U.S. forces in Iraq has failed to corroborate the threat.

Two unnamed law enforcement officials have told The Associated Press that the suspects in custody in Iraq insist they never planned to coordinate attacks with operatives in New York.

They have even passed polygraph tests on that very question.

"The people supposedly standing by in New York probably were never there," one of the officials told AP, speaking on condition of anonymity.

On Monday, New York City Police Commissioner Raymond Kelly announced that random bag searches and other precautions would remain in place, but security on the system would be gradually scaled back.

Responding to suggestions city officials had overreacted, Kelly told reporters the tip had not only come from a reputable informant, but was also too specific to ignore.

"We did precisely the right thing," Kelly said. "We had no choice but to respond the way we did."

When officials couldn't corroborate the key informant's claims, they redressed the question. At that point, citing government sources, CNN reports the informant admitted having made the whole thing up.