The FBI is warning police that terrorists could construct a simple but deadly chemical weapon out of materials readily available.

"Little or no training is required to assemble and deploy such a device due to its simplicity," the FBI said Wednesday in its weekly intelligence bulletin to about 18,000 law enforcement agencies.

The bulletin provides no details of a specific threat or possible location of an attack. It does say that terrorists could take advantage of building ventilation systems, air intakes or enclosed areas to disperse toxic chemical gas.

Law enforcement officials previously have warned that Al Qaeda or other terrorist groups might target subways and targets such as hotels and office buildings rather than heavily guarded government installations.

In addition, material collected in Pakistan after the March 1 capture in that country of senior Al Qaeda planner Khalid Shaikh Mohammed provided further proof that operatives experimented with various forms of chemical, radiological and biological weapons, law enforcement officials say.

The FBI bulletin says hydrogen cyanide or chorine gas could be produced by combining liquid and solid materials, possibly using a canister such as a paint can with holes pierced into it. The materials could be combined using either a blasting cap or some kind of delayed switch.

"When combined, this creates the toxic gas that would emerge through the holes," the bulletin says.

Such a device would be most effective in an enclosed space, the bulletin adds, because it would be dispersed too quickly in larger areas or out in the open to kill or injure many people. But police, firefighters and medical personnel could be imperiled when responding to an attack because "the device may reactivate when it is disturbed."

In January, the FBI told police to beware of possible attacks using ricin, a toxic substance derived from the castor bean plant. That warning followed the arrests last year in Britain of 11 North African men on terrorism charges stemming from an alleged attempt to develop a ricin weapon.

The bulletin came as the FBI continues to interview Iraqis living in the United States at the rate of about 1,000 a day, with a goal of reaching 11,000 by the end of this week.

The interviews, focused on those who have recently traveled to Iraq or have ties to the Iraqi military, are intended to discover the identities of any terrorists and spies in the United States and also to find any information that might be helpful to U.S. forces in Iraq.

The bulletin also repeated the FBI's search for Adnan G. El Shukrijumah, a 27-year-old Saudi-born man who may be an Al Qaeda operative. El Shukrijumah left the Miami area in May 2001 for Morocco, according to his family, but law enforcement officials say they do not know his whereabouts.

El Shukrijumah was identified in part by information collected after Mohammed's capture in Pakistan.

In addition, the bulletin asks police to look for Dr. Mohammed Khan, 33, and his estranged wife, 31-year-old Aafia Siddiqui, both of whom the FBI wants to question about possible terrorist ties.

Siddiqui, who has a doctorate in neurological science from the Massachusetts Institute of Technology, may be in Pakistan, FBI officials say. She lived in Boston while attending MIT and also recently traveled in Maryland.

The bulletin provided no further information about Khan.