A hydrogen cyanide attack in the subways, like the plot that was reportedly planned against New York City in 2003, is virtually unpreventible, counterterrorism and chemical weapons experts said yesterday. And with fears that the Al Qaeda cell that came here to carry out the attacks is still in the country, some are worried the subways are an open target.

The 2003 plot is detailed in journalist Ron Suskind’s new book, “The One Percent Doctrine: Deep Inside America’s Pursuit of Its Enemies Since 9/11,” published today. The book states that Al Qaeda operatives came to America to carry out the attack, but the group’s second-in-command, Ayman al-Zawahiri, called it off just 45 days before deadline.

Mr. Suskind speculated in an interview with Time magazine that the attack was called off because Al Qaeda leaders didn’t think it would be a sufficiently traumatic “second wave” following the September 11, 2001, attacks.

The device developed by Al Qaeda – referred to as the “mubtakkar,” or “invention” in Arabic – is described by Mr. Suskind as two mason jars connected by a glass tube with a remote control seal. Experts said such a device would be easily concealable, even with random searches on the subways.

A hydrogen cyanide attack would have a low number of casualties, but a high level of terror, according to the director of the Center for Nonproliferation Studies and former U.N. weapons inspector in Iraq, Jonathan Tucker.

The psychological effect of even a few deaths would be devastating, experts said. The proof of such a reaction is in Tokyo, 1995, when the cult Aum Shinriyko killed 12 people on the subway system with sarin gas, a nerve agent. Several thousand were hospitalized, including a handful that suffered permanent paralysis.

Mr. Tucker was hired by the government to analyze the Al Qaeda training videos of chemical weapons testing on dogs discovered during the war in Afghanistan. His analysis showed that the gas used to kill the dogs in the glass boxes was hydrogen cyanide. Those videos were an early sign that the loose terrorist organization was earnestly planning to use chemical weapons in an attack, he said.

The chemicals needed to create hydrogen cyanide are prevalent throughout industries that deal with metals and plastics, as well as places like hospitals, photo developing shops, and high school chemistry labs. Merely mixing the two substances together creates the deadly gas.

The cyanide attacks the mitochondria in cells, causing them to run out of energy and fail, a medical toxicologist at New York University School of Medicine and researcher at New York University’s Center for Disaster Preparedness and Response, Lewis Nelson, said. The symptoms run the gamut of serious medical emergencies: seizures, cardiac arrests, coma, and with a serious exposure, death within minutes.

The city has stepped up its ability to respond to hazardous material attacks since the September 11 attacks. The Fire Department now has 35 “Haztac” vehicles, which carry antidotes to chemical weapons, including cyanide.

The Police Department began random searches of bags at some subway stations last summer. Transit police also use a tactic called Total Order Maintenance, where a dozen officers line up along the platform and enter every subway car as they pull in. And the Metropolitan Transportation Authority has begun an ambitious program to equip the subways with hundreds of cameras and counter-terrorism technology.

The mayor said yesterday that he and the police commissioner, Raymond Kelly, were aware of the threat but chose not to make it public.