The unidentified gas used by Russian security forces in their raid on a Moscow theater appears to have been an incapacitating agent that may fall into the gray area of international restrictions on chemical weapons, U.S. experts said yesterday.

Before storming the theater, where about 700 people were held hostage by Chechen militants, security forces pumped an odorless gas into the building's ventilation system that put most of the hostages and their captors to sleep.

Russian officials have declined to identify the chemical used in the operation, describing it generically as a "sleeping gas" or "special gas."

"We have only been given general information that it was an incapacitating or calming agent, but we do not know specifically the nature of the substance," U.S. Ambassador Alexander Vershbow told reporters in Moscow.

Emergency workers who entered the theater after the raid reported seeing people slumped over as if they were sleeping. There were some reports of nausea and vomiting, along with hallucinogenic effects.

Experts said it was impossible to know for sure what gas was used without more detailed evidence of its effects, but speculation included an aerosol form of Valium or a Cold War-era agent called BZ, which was developed by the United States and nicknamed the "sleeping agent" by U.S. soldiers. BZ can produce both sleepiness and hallucinations.

"It sounds like some sort of incapacitating agent, and BZ certainly fits in that category because it can put you to sleep," said Frederick Sidell, a former U.S. Army chemical weapons expert.

But some experts also cautioned that BZ is highly unpredictable and frequently increases agitation and excitability, which would have undermined the goal of neutralizing the militants.

Jonathan Tucker, a longtime chemical weapons expert and a senior fellow at the U.S. Institute of Peace in Washington, said that the use of BZ or other similar chemicals would be a violation of the Chemical Weapons Convention, which was signed by Russia in 1997.

The convention bans the production, stockpiling and use of numerous toxic chemicals and agents, but includes exceptions for nonlethal agents used for riot control and other domestic security purposes.

"There's something of a blurry line between a riot-control agent and an incapacitant," Tucker said. "Something like tear gas, which has a very transient effect, is allowed, but an agent that has incapacitating effects for several hours are clearly banned."

Elisa Harris, a chemical weapons expert at the University of Maryland and a former staff member at the National Security Council, said that "if they used something other than tear gases in this scenario, they may well be in violation of the Chemical Weapons Convention. But there's really too little information at this point to tell for sure."

Lev Fedorov, president of the Russian Union for Chemical Safety, speculated that the gas used was one not banned by the convention.

"Many countries have such poisonous substances, including Russia," he said on Russian television. "With us, they came into being around the threshold of the 1960s and the 1970s and thank God were never used on such a big scale. But this time around one had to use it."

Correspondent Peter Baker in Moscow contributed to this report.