New York Mayor Michael Bloomberg says he was erring on the side of caution when he went public Thursday with a threat that terrorists might attack the New York subway system.

Homeland Department officials in Washington Friday downplayed the threat, saying that while it was specific it was of "doubtful credibility."

New York City Mayor Michael Bloomberg rides the subway, Oct. 7. (Photo by Bryan Smith-Pool/Getty Images)

"If I'm going to make a mistake you can rest assured it is on the side of being cautious," Bloomberg said at a news conference. He stressed he did not think any mistake had been made.

New York Police Commissioner Ray Kelly agreed with the mayor. "We did exactly the right thing," Kelly said.

Bloomberg had asked New Yorkers to leave bags, briefcases and kids' strollers at home, but did not advise them to abandon the subway and said he'd be taking it himself.

The New York officials who raised the alarm Thursday said they had been informed of the threat by a source at the Federal Bureau of Investigations. They said the threat was uncovered during a raid in Iraq Wednesday where at least two people were arrested.

Details of the threat were very specific, including the possibility of bombs being hidden in strollers.

New York police increased their presence on the subway, and searched bags for bombs.

New York normally is on a high state of alert, going back to the devastating World Trade Center attacks of Sept. 11, 2001.

Meanwhile, some tense moments at Pennsylvania Station on the subway Friday followed discovery of a discarded soda bottle filled with an unidentified green liquid.

Penn Station was closed for an hour, but the liquid turned out to be a kind of drain cleaner and posed no danger to commuters.

President George Bush said the New York officials exercised their own prerogative in publicizing the threat Thursday.

"I think they took the information we gave and made the judgments they thought were necessary," Bush said.

An estimated 4.5 million people ride the New York subway rails on an average weekday. The system has more than 468 stations.

Random searches have been done on the subway since July of this year when terrorist attacks in the London subway killed 56 people.