Washington ? Russia’s fatal use of an incapacitating gas to end a hostage crisis raises questions about whether the United States should continue looking for such means to calm large crowds or abandon the research altogether.

U.S. military and diplomatic officials said Monday the gas that killed more than 100 of the Moscow hostages was an opium derivative part of a class of drugs that researchers suggested two years ago the Pentagon should investigate for development as nonlethal weapons.

Russian authorities have refused to identify the substance used, even to doctors treating the freed hostages. Hundreds remained hospitalized Monday, including more than four dozen in critical condition.

U.S. officials believe the gas was an opiate, not a nerve agent. Opiates, a class of drugs including morphine and heroin, not only kill pain and dull the senses but also can cause coma and death by shutting down breathing and circulation.

Supporting the theory that an opiate was used was that Russian doctors treating the hostages told U.S. Embassy workers they tried atropine an antidote to many nerve agents and that didn’t work, a Bush administration official said.

However, a drug that reverses the effects of opiates, Narcan, did appear to help, the official said.

A bus filled with unconscious freed hostages leaves the theater that was seized by Chechen rebels in Moscow. The deadly effects of the gas used to incapacitate the rebels raises questions about pursuing the use of crowd-calming agents.

Nevertheless, some medical experts questioned whether opiates were involved. Unless Russia has some secret chemical weapon, the only substance that could incapacitate people that quickly would be a form of nerve gas, said Dr. John Tinker, head of the anesthesiology department at the University of Nebraska Medical Center in Omaha.

The victims could have appeared to be unresponsive to atropine because the chemical is only a partial antidote and has no further effect after it reaches a certain level in the body, Tinker said.

U.S. military research into so-called “calmative” agents is on hold amid worries such weapons would violate the international treaty banning chemical weapons. However, the Justice Department has given $35,000 to Pennsylvania State University researchers investigating whether a calming drug could be added to pepper spray to make a better riot-control agent.