MOSCOW, 30 October — Doctors who examined victims of the Moscow theater siege believe they were knocked out by an opiate rather than nerve gas, American officials said yesterday.

Officials at the US Embassy in Moscow said the doctors said the effects of the gas appeared consistent with an opiate such as a heroin derivative. Such substances not only kill pain and dull the senses but can also cause coma and death by shutting down breathing and circulation.

But as the Russian military continued to refuse to confirm the components of the gas used to end the weekend siege, despite repeated formal requests, other doctors remained mystified by the effects.

"It remains a puzzle," said Dr. Thomas Zilker, a toxicology professor at Munich University Clinic in Germany, who is treating two former hostages. He said the gas did not appear to be a known chemical weapon and could be a secret formula developed by Russia.

The Russian government will not reveal the type of gas because it may face similar attacks in the future, according to Alexei Arbatov, a member of Russia’s lower house of the Parliament. He described the substance as a "police gas" used to incapacitate the rebels. "In order to be able to use such gas again, the secret could not be disclosed," he said.

Police arrested dozens of people suspected of involvement in the three-day standoff that left 117 hostages and 50 Chechen militants dead, almost all of them gassed. Officials said that 41 of the Chechen hostage-takers had been shot dead during the rescue operation. Many of them were unconscious at the time from the effects of the gas, according to members of the special forces who led the assault.

A member of the special forces told Kommersant newspaper that the Russians had shot dead those Chechens who had explosives strapped to their bodies even though they were already knocked out by the gas, to avoid them triggering their human bombs.

Interior Minister Boris Gryzlov said several dozen people believed to have participated in the theater siege had been arrested. A Chechen lawmaker complained that Chechens in the Russian capital were being harassed, searched and taken in for photos and fingerprinting.

A Russian helicopter attempting to land near the military’s main base in Chechnya was shot down by a missile, killing all four aboard in the latest of a series of helicopters apparently downed by rebels in the breakaway region. The Mi-8 helicopter was 30 meters (100 feet) off the ground when it was hit, caught fire and fell in an explosion near Moscow’s main military base in the region at Khankala, said Alexander Lemeshev, an official at the Emergency Situations Ministry’s southern Russia branch.

According to preliminary information, three crew members were aboard with one passenger, an officer who was accompanying some cargo, Lemeshev said.

Russia has ruled out all talks with Chechen leader Aslan Maskhadov, who said Monday that more attacks like the Moscow hostage taking were inevitable unless Putin sought a peace settlement. He firmly denied any involvement in the audacious attack on the Russian capital, however. (The Independent & Agencies)