The New York Police Department and FBI are investigating a "credible" tip that 19 operatives have been deployed to the city to place bombs in the subway, and security in the subways has been increased. Department of Homeland Security sources told ABC News they were very doubtful the threat information is credible, however.
The city's police department said it was taking the threat seriously and believed the source was reliable, but also urged the public not to be alarmed because the information had not been verified.
Mayor Michael R. Bloomberg said this was the most specifically detailed threat made against the subway system, and he urged New Yorkers to be vigilant.
"I wanted to assure New Yorkers that we have done and we will continue to do everything we can to protect the city," Bloomberg said. "We will spare no resource. We will spare no expense."
According to sources in intelligence, emergency services and police headquarters, the intelligence community developed information that the threat may have involved pharmacists from Iraq coming to New York for some kind of chemical attack targeting the subways.
Three insurgents, one or more of whom are pharmacists, were arrested during a raid by a U.S. military and intelligence community team, sources said, and one of those caught disclosed the threat. Because it slipped out during the arrest, the plot was deemed credible.
After several days of work, sources said, the NYPD became increasingly concerned because it was unable to discredit the initial source and additional information from the source.
The 19 operatives were to place improvised explosive devices in the subways using briefcases, according to two sources. Police Commissioner Ray Kelly said officers will continue to check bags, briefcases, luggage and strollers, and additional uniformed and undercover officers will be riding in individual subway cars.
The police are deploying additional officers, dogs and heavy weapons teams in subways and commuter rail terminals, sources said.
Rep. Peter King, R-N.Y., chairman of the House Homeland Security Committee, told The Associated Press, "Obviously, this is a significant threat."
Mark Marshon, assistant director in charge of the New York office of the FBI, said the investigation has helped stop the plans. "The encouraging news is that classified operations have, in fact, partially disrupted this threat," he said.
Bloomberg added that he will continue to ride the subways.
ABC News' Rich Esposito and Pierre Thomas contributed to this report.