Police in New York City subway station

Authorities in New York City have increased security in the subway system after receiving a specific threat of a terrorist attack. Officials revealed few details of the threat, other than that it came from overseas.

New York City Mayor Michael Bloomberg announced the heightened security measures at a hastily called late afternoon news conference. Flanked by New York City Police Commissioner Raymond Kelly and Assistant Director of New York F.B.I. office Mark Mershon, the mayor called the measures a precaution in the face of a specific threat, as well as the recent history of attacks in the United States and Europe.

"As we have known since 9/11, and even more since the Madrid and London attacks, our mass transit system is a potential terrorist target. Since the London attacks, the NYPD has taken additional measures to ensure the security of New Yorkers. The F.B.I. has recently shared with us a specific threat to our subway system," he said.

Mayor Bloomberg said he would continue to ride the subway to work, as he regularly does, but warned that security measures, including thorough bag checks, would be sharply increased. He urged New Yorkers to take extra precautions, and to report any suspicious activity.

Police Commissioner Kelly said the city's security alert level remains at orange, the level it has been at since the attacks of September 11, 2001.

The F.B.I's Marc Mershon revealed that the information about the threat was linked to the arrest of three persons in Iraq. He said the F.B.I. had urged city officials to give the threat unprecedented attention.

"The detail of this specific threat is in fact classified. We put down threats, multiple threats, every day. But the detail of this specific threat was so "on point" that we did raise this concern with the New York City Police Department," he said.

Mr. Mershon gave few details, but said the threat had been partially disrupted by law enforcement operations. Officials said no one has been arrested or detained in New York.

The New York City subway system is one of the world's largest. It has more than 450 stations, more than 1,000 kilometers of track, and serves 4.5 million passengers on an average weekday.