The New York Police Department has quietly begun tracking chlorine shipments in the city and requiring increased security at some storage areas in response to terrorists' use of the chemical weapon in Iraq, police officials said.

In Ramadi yesterday, a tanker full of chlorine gas was exploded near a restaurant, killing as many as six people and injuring 10 others, the Associated Press reported. Twenty-seven people were killed in a similar attack in the Iraq city on April 6.

The police department's chief spokesman, Deputy Commissioner Paul Browne, said the department's analysts several months ago started looking at chlorine. As the attacks in Iraq became more frequent, officers in the counterterrorism and intelligence divisions began taking a closer look at chlorine in the city.

Mr. Browne said officers have also been stopping vehicles transporting chlorine to check if they are properly licensed. At a recent New York Shield briefing — a periodic meeting where private security directors from around the New York metropolitan area get updated on potential threats against the city — police officials described the nature of a chlorine threat, he said.

Detectives regularly visit locations around the city to reduce the risk of certain chemicals or materials getting into the hands of a terrorist as part of what is called Operation Nexus, officials said. They visit hospitals that have equipment with radioactive materials, industrial storage areas, and truck rental agencies, among other locations. Business owners are asked to call in tips about suspicious activity.

"NYPD has been concerned about dangerous materials that are stored in or transited through the city," a former head of the department's counterterrorism division, Michael Sheehan, a fellow at New York University's Center on Law and Security, said. "NYPD intelligence analysts also closely track international trends in counterterrorism in order to anticipate what types of threats could be manifested in New York City, such as the use of chemical weapons in Iraq."

He added: "This most recent action is part of a much broader strategy — a local nonproliferation strategy — implemented by Commissioner Kelly in an attempt to pre-empt chemical, biological, or radiological threats before they get organized in the city."

The infrastructure protection chief of the Department of Homeland Security, Robert Stephan, said last week that authorities should step up their scrutiny of chlorine across the country, adding that 150-pound containers of chlorine are in wide commercial use throughout the country.

"We've got to be prepared for it," he told USA Today. The paper reported that the Chlorine Institute, a trade group for the industry, recently alerted the FBI to stolen chlorine tanks in California. A 1,600-gallon tank of chlorine disappeared from a parking lot in Tallahassee, Fla., early last month, the Tallahassee Democrat reported.

Chlorine is widely used in water purification and sewage plants, which is where Iraqi insurgents are thought to have been stealing tanks from in recent months.

In small concentrations chlorine gas causes coughing, blurred vision, nausea, burning pain, and makes it difficult to breathe, according to the Centers for Disease Control and Prevention. When the chemical comes into contact with water, it produces acid, which makes the eyes and respiratory system especially vulnerable. Higher concentrations can cause death.

According to a U.S. Coast Guard study, a chlorine gas cloud can spread two miles in 10 minutes.

"The people are here, the knowledge can get here, the materials are here," the director of the Manhattan Institute's Center for Policing Terrorism, Timothy Connors, said. "It's used in a lot of industrial applications, huge chemical plants, and it's stored in train cars. … If it could be put together in Iraq, it could be put together in New York."

There are no specific terrorist threats against the city, Mr. Browne said.

Journalist Ron Suskind, in his book "The One Percent Doctrine: Deep Inside America's Pursuit of Its Enemies Since 9/11," said Al Qaeda operatives planned an attack using hydrogen cyanide gas in the subways in 2003, but called it off 45 days before the proposed deadline.
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