New York Mayor Michael Bloomberg has again defended last Thursday's decision to issue a high alert for the city's underground train system.
Michael Bloomberg says the threat was credible
Mr Bloomberg, who insisted the city did not overreact, was speaking after US media reported that the alleged subway threat was a hoax.
"I don't know that it was confirmed that it was a hoax at all," he said.
Extra security measures were scaled down on Monday, after officials said the period of the threat had passed.
But on Tuesday, US reports cast doubt on the credibility of the threat.
They said the informant who had told US officials about the plan to attack the subway with briefcases and pushchairs packed with explosives had later admitted to making it up.
They also said that the three suspects he identified in Iraq appeared to have no links to any plan or known extremist groups.
Mr Bloomberg said the informant had passed a lie detector test, the Associated Press reports.
The FBI and White House officials had earlier expressed their doubts about the alleged threat, which was said to involve more than a dozen operatives in Iraq and the US.
But Mr Bloomberg defended his actions, saying: "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."