WASHINGTON – As the FBI warns police across the country that al-Qaida continues to show interest in attacking U.S. subways, new intelligence has emerged that al-Qaida has trained operatives to dispense poisonous gases such as cyanide in ventilation systems.

Buried in one of the recently released staff reports by the 9-11 Commission is a chilling account of some of the experimentation in methods of mass murder conducted at al-Qaida camps.

"The camps created a climate in which trainees and other personnel were free to think creatively about ways to commit mass murder," according to Staff Statement No. 15, titled "Overview of the Enemy."

"According to a senior al-Qaida associate, various ideas were floated by mujahidin in Afghanistan," the report added, including "mounting mustard gas or cyanide attacks against Jewish areas in Iran" and "dispensing poison gas into the air conditioning system of a targeted building."

The disclosure by the al-Qaida agent may explain why the FBI has warned law enforcement authorities to be on alert for signs of a biological or chemical attack against the homeland by al-Qaida terrorists.

Unexplained or unusual odors, such as the smell of "bitter almonds," are among "potential indicators of threats involving" chemical weapons such as cyanide gas, according to a security advisory issued in late December by the FBI counterterrorism division here. WorldNetDaily obtained a copy of the report labeled "law enforcement sensitive."

An earlier U.S. homeland security report, dated Nov. 21, said the terrorists have designed a "crude chemical dispersal device" fabricated from commonly available materials, which is designed to asphyxiate its victims, as WorldNetDaily also first reported. The device can produce both cyanogen chloride gas and hydrogen cyanide gas, which gives off a bitter almond smell, the report said.

"These gases are most effective when released in confined spaces such as subways, buildings or other crowded indoor facilities," the closely held report said, adding that al-Qaida has shown a continued interest in targeting subways.

"Al-Qaida remains intent on using chemical or biological agents in attacks on the homeland," it said.

Washington and New York operate the largest rail transit systems in the country. The FBI fears al-Qaida plans to attack before the November election, and a likely target may be the GOP convention in New York City, which begins in late August.

To guard against such attacks, the agency is advising law enforcement and security personnel to be on the lookout for suspicious packages or containers, as well as "unusual powders or liquids" near air-intake systems or enclosed spaces, the December advisory said. Discovery of unusual spray devices or bottles is also a priority.

In addition, it lists several odors that are telltale signs of a chemical attack.

"Smells may range from fruity/flowery to sharp/pungent, garlic/horseradish-like, bitter almonds, peach kernels and new-mown grass/hay," according to the FBI advisory.

The bitter almonds smell is associated with cyanide gas. An antidote for cyanide poisoning is hydroxocobalamin, which is administered in very high doses, a U.S. official told WND.

The new-mown hay smell is a signature of phosgene gas, also known as carbonyl chloride, according to the official. The odor is mild and there are no immediate symptoms to exposure. Later on, however, the lungs fill with fluid as the material breaks down into chlorine and hydrochloric acid.

The fruity smell, almost like green apples, is associated with CN, or riot gas, the official said.

Though there is no specific threat timed to the Independence Day holiday, homeland security officials urged Americans to increase their vigilance during the extended weekend.

"We know the U.S. homeland remains a top al-Qaida target," warns the new security advisory, which was sent this week to 18,000 state and local law enforcement agencies.

Previous stories:

New FBI warning cites biochemical threat

Al-Qaida to gas U.S. subways?