The United States criticized Russia today for keeping secret the type of gas used to end last week's theater hostage crisis, as indications increased that some or even most of the 115 civilians killed by the gas might have been saved if medical crews had been prepared.

As Moscow began to bury its dead, U.S. Ambassador Alexander Vershbow said Russia had still not disclosed to him the gas used in Saturday's commando raid but confirmed that American doctors had determined it was a form of opiate similar to fentanyl. The failure to inform Russian doctors who treated the hostages may have cost lives, he said.

"We regret that the lack of information simply contributed to the confusion after the immediate operation to free the hostages was over," Vershbow told a briefing at Spaso House, the ambassador's residence here. "It's clear that perhaps with a little more information at least a few more of the hostages would have survived."

Vershbow's comments deviated from the congratulations he and other U.S. officials offered when the death toll was still not clear in the hours immediately after Russian special forces stormed a theater where Chechen guerrillas had taken 800 people hostage. All but two of the 117 hostages killed died from the gas, according to Moscow medical authorities. Among the dead were an American citizen and a Ukrainian with a U.S. green card.

A Russian doctor who has been treating the hostages at a Moscow hospital constantly since Saturday said the failure to prepare medical rescue units led to many of the deaths. Agreeing that the gas appeared to be an opiate, the doctor said "it's a fairly harmless substance" if used correctly, but the rescue crews that first arrived on the scene "weren't prepared for detoxification."

"If people were intubated and helped to breathe with artificial ventilation while still in the vehicles being brought to the hospitals, almost everyone would have survived," the doctor said in an interview tonight on the condition that he not be identified for fear of reprisals. The doctor blamed Moscow city health officials for not agreeing to accept additional resources from the federal government, such as paramedics and medical equipment.

All of those who died, he added, were already dead by the time they arrived at the hospital: "Everyone brought to my hospital alive is still alive."

The ripple effects of the hostage crisis continued to play out across Russia today, from the cemeteries where the first funerals were held to the State Duma where lawmakers debated a response and to Chechnya where rebels shot down an Mi-8 military helicopter, killing four aboard.

Police fanned out across Moscow today in search of other guerrilla cells lying in wait and rounded up dozens of Chechen men. Some had traces of explosives on them while police found two homemade explosive devices and a plan of a railway station, officials told Russian news agencies.

Investigators labored to figure out how at least 50 heavily armed Chechen militants made it to Moscow and into the theater where "Nord-Ost" ("North-East"), a popular musical, was playing last week. Officials are exploring whether a car bombing at a McDonald's restaurant four days earlier might have been related. There have also been conflicting reports in the Russian media as to whether a Russian police officer had been arrested on suspicion of collusion.

"Today the Interior [Ministry] forces are conducting unprecedented actions aimed at exposing a terrorist network in Moscow and the Moscow region," Interior Minister Boris Gryzlov announced. "We have already detained several dozen men suspected of involvement with the hostage-taking."

Moscow remained skittish. A theater where the American musical "42nd Street" is playing was evacuated this evening by a bomb threat. And the director of "Nord-Ost," which lost 17 cast and crew members, announced today that his musical would close. "Even if the Moscow administration reconstructs [the theater]," Georgi Vasilyev told Echo Moskvy radio, "this place will still remain accursed."

While focusing on the hunt for Chechen militants, authorities maintained silence about the gas they used to subdue the hostage takers at the House of Culture for the State Ball-Bearing Factory. Western physicians who have examined freed hostages have concluded it was a morphine-like opiate, probably a gas version of fentanyl, which is used as an anesthetic.

As 16 victims were laid to rest today, another 317 freed hostages remained in hospitals recovering from the after-effects of the gas, including seven children. Twenty-seven were listed in critical condition.

Physicians remained hamstrung in helping their patients as the government continued to withhold any information. "The patients are currently being treated with antibiotics as a way of preventing potential pneumonia," Nellogi Saakyan, head of surgery at Moscow's Hospital No. 13, the main treatment center for freed hostages, said today. "We are not familiar with the specific substances used."

According to U.S. experts, fentanyl belongs to a group of medicines called narcotic analgesics that suppress breathing. While similar to its cousins morphine and Demerol, medical specialists said fentanyl is 100 times more potent than morphine.

A normal dosage goes to the brain but is then redistributed quickly to the rest of the body, making it a short-acting anesthesia. But a larger dose does not redistribute as well, remaining concentrated in the brain and shutting down respiratory functions.

"It just turns off your desire to breathe," said Barry L. Friedberg, an anesthesiologist in Newport Beach, Calif., and fentanyl critic. "It takes three to five minutes of not breathing to have death from lack of oxygen. . . . Once you head into that downward spiral, you can't be resuscitated."

Friedberg said fentanyl is normally administered as a liquid, either by drinking or injecting, and he had never heard of a nebulized or gas version. The drug naxolone counteracts its effects but would have had to be administered within minutes by Russian rescue workers who were never informed what medications to bring with them.

The Kremlin dismissed questions about its decision to pump gas and storm the building. "The [command] headquarters did not have a single scenario which would guarantee survival of the hostages and special forces given that 150 kilograms [330 pounds] of explosives had been placed inside the theater," Kremlin aide Sergei Yastrzhembsky said.

In his remarks, Vershbow stressed that the United States still backed Russia's decision to take action and only questioned its conduct afterward. "They had a difficult decision to make. With the bombs that were there they probably saved hundreds of lives, even though we regret that more than 100 died."

However, he added that Russia still needs to address the greater problem of the war in Chechnya. "We continue to see the Chechen conflict at its roots as an internal separatist movement that has been in some ways exploited by international terrorist groups that see this as part of their global agenda," Vershbow said. In staging terrorist acts, the Chechens "compromise their own cause. They've crossed a line and are carrying out acts that we can only condemn."

Officers carry the coffin of Col. Konstantin Litvinov, 50, a hostage. Some suggest better-informed medical crews could have saved more victims of the gas.A relative grieves at Col. Litvinov's funeral. U.S. doctors believe the gas used to end the crisis is an opiate.A woman passes the theater where 117 civilians were killed in last week's hostage crisis. A Russian doctor said that, if victims had been intubated on the way to the hospital, "almost everyone would have survived."