Mayor Michael Bloomberg and police officials questioned reports on Tuesday that an alleged subway bomb plot that spread fear through the city was a hoax.

"I don't know that it was confirmed that it was a hoax at all," Bloomberg said when asked about the reports by a cable news network and the New York Post, which is owned by News Corp., the parent company of FOXNews.com.

The reports, citing unnamed sources, claimed an informant in Iraq who had told U.S. authorities about the possible threat by Al Qaeda (search) later admitted he made it up.

The informant's allegation prompted city and FBI (search) officials last week to issue a dire public warning and to flood the subway system with thousands of extra police officers.

After four days of high alert, the officials announced Monday there was no clear evidence an attack would be carried out and scaled back the protection.

Police Commissioner Raymond Kelly (search), appearing with Bloomberg at a hospital groundbreaking in Harlem, said the informant could not have recanted because U.S. authorities in Iraq had lost track of him.

"As far as we know right now, the source is not in custody," he said.

Bloomberg said the informant had passed a lie detector test at the time he identified three suspects in Iraq, and alleged they were plotting to use other operatives in New York to attack the subway with briefcases and baby carriages packed with explosives. Officials have also said the informant had provided reliable information in the past.

Kelly said he participated in an interagency conference call on Thursday before issuing the public warning, and during the meeting he learned that one of the alleged plotters had indicated it was "too late" to prevent the attack.

"Certainly it was a factor," Kelly said. "We had to make a decision quickly."

The mayor continued to insist the city didn't overreact.

"When somebody makes an allegation, and it looks like there is some credibility — and clearly there was credibility here — you go and you act on it," he said.

A call to the FBI's New York office was not immediately returned.