New York Mayor Michael Bloomberg has been defending his decision to put the city on high alert over a threat to the subway system.
Michael Bloomberg took the subway to work on Friday as usual
Asked if he had acted too hastily, Mr Bloomberg said that he had made the right decision.
Security was stepped up on the subway after city officials said they had received a "credible security threat".
But in Washington, White House spokesman Scott McClellan said the threat was "of doubtful credibility".
Homeland Security spokesman Brian Doyle said that the threat information was checked out by intelligence agencies, which "couldn't put a credible factor on it".
President George W Bush defended Mr Bloomberg, saying he would not second-guess the mayor's move.
Police patrols and searches in New York are being stepped up, and travellers have been urged to avoid using bags, briefcases and pushchairs.
Asked whether he had made the right decision by putting the city on high alert, Mr Bloomberg said he had a responsibility to protect people's lives and that he would make the same decision again.
Security checks are increasing on the New York subway
"If I'm going to make a mistake, you can rest assured it is on the side of being cautious," Mr Bloomberg said.
"It is very different being an analyst in Washington looking at data as opposed to being here in New York where you have to take responsibility to protect people's lives," he added.
The mayor announced on Thursday that the authorities had received information that the subway system could be the subject of a terrorist attack, saying the threat was the most specific yet.
Officials said details of the threat were classified, however Mr Bloomberg said it came from overseas.
An unnamed intelligence official told the Reuters news agency that the information came from people recently captured in Iraq by the US military.
An estimated 4.5m people use the New York subway every weekday.
The system has more than 468 subway stations.
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