MOSCOW - The gas used by Russian commandos in their assault on a Moscow theater taken over by Chechen militants killed all but two of the 118 hostages who have died so far, Russian health officials said last night, casting a shadow on what was first seen as a triumphant rescue operation.

Nearly 650 former hostages remained hospitalized, with 45 in critical condition and 105 in intensive care, said Moscow's chief physician, Andrei Seltsovsky.

The conclusion that nearly all the slain hostages died from the gas and not from their captors' bullets contradicted initial assertions by law-enforcement officials.

The medical findings, and new accounts from former hostages, also indicated the Chechens had not begun systematically killing prisoners as Russian authorities believed before launching the assault early Saturday morning.

Yevgeny Luzhnikov, head of Moscow's Department of Severe Poisoning, said troops did not tell medical authorities they had gassed the auditorium until the 750 hostages were brought out, most of them unconscious. "But we didn't know the character of the gas," he said.

The substance was described as akin to compounds used in surgical anesthesia. Seltsovsky said it affected the heart and lungs. He said he had no information when asked about reports that the compound could cause vomiting that would choke unconscious victims.

"In standard situations, the compound... does not act as aggressively as it turned out to do," Seltsovsky said. "But it was used on people who were in a specific [extreme] situation for more than 50 hours... . All of this naturally made the situation more difficult."

The White House declined to criticize the rescue operation, making clear the Bush administration's view that blame for the deaths lay with the captors.

"The Russian government and the Russian people are victims of this tragedy, and the tragedy was caused as a result of the terrorists who took hostages and booby-trapped the building and created dire circumstances," spokesman Ari Fleischer said yesterday.

A group of 54 Chechen militants, 50 of whom were killed in the siege, took over the theater Wednesday night during a musical. They seized about 800 people, some of whom later were released or escaped. The Chechens demanded the immediate withdrawal of Russian troops from Chechnya and threatened to blow up the building.

In the predawn raid Saturday, Russian troops pumped the gas into the theater before storming it.

There were numerous, unverified reports of semi-conscious hostages having choked to death on their own vomit inside the theater due to the gas attack. Seltsovsky and other health officials said yesterday that most had died in various hospitals from respiratory distress and heart failure.

Russian officials declined to specify the exact name of the gas. Foreign diplomats have demanded information about the gas from the government, but without reply.

An estimated 70 foreign citizens were among the captives. A U.S. Embassy official said an American woman and a U.S. green-card holder had finally been located in local hospitals, although he gave no personal details. Consular officials continued to search for at least one other American believed to have been a hostage.

At least two foreigners - one Dutch and one Austrian - died.

The gas was powerful and fast-acting enough that the Chechens were unable to detonate the numerous bombs and mines they had placed around the theater.

One former hostage described the gas as bluish-gray, and another said it was bitter-smelling. The Interfax news agency quoted an unnamed ex-hostage as saying he saw one of the militants trying to put on a gas mask as the attack unfolded.

"He made several convulsive moves, trying to pull the mask over his face, and fell," the ex-hostage said.

Yesterday afternoon, more than 24 hours after the hostages had been freed, hospitals posted lists of patients. Visitors were still prohibited. Some people outside the gates saw their relatives waving from windows.

"They are hostages again," one visitor shouted to the armed guards at Hospital 13, where about half the former captives were taken.

Most who left the hospitals hugged those meeting them, then hurried to get out of the chilling rain and avoid reporters and TV cameras. Some former hostages contradicted the official version of events.

Mark Podlesny and Georgy Vasilyev, producer of the musical, Nord-Ost (Northeast), disputed Russian officials' statement that the Chechens had begun shooting hostages before dawn and had prompted the special forces' assault.

Podlesny also questioned Russian television footage that showed the captors' corpses in the theater amid liquor bottles and syringes. "They didn't drink, didn't smoke, didn't swear. They were very disciplined," he said.

A total of 118 hostages were known to have died after the Chechens stormed the theater - 116 from the effects of the gas, one young woman shot early in the standoff, and one hostage shot Saturday morning shortly before the raid.

President Vladimir V. Putin declared today a national day of mourning, with flags to be flown at half-staff. Putin also asked TV networks, radio stations and cultural centers to cancel their entertainment programs.

Many of the 50 assailants killed in the rescue mission died after being shot in the head, apparently while unconscious from the gas.

The chief Moscow prosecutor, Mikhail Avdyukov, said three people had been arrested in the city on suspicion of helping to carry out the raid, Interfax reported.

The attackers included 18 women, many of whom said they were war widows. The women strapped explosives to their bodies, and mines were placed throughout the building. The attackers had threatened to blow up the theater unless Putin agreed to withdraw troops from mainly Muslim Chechnya.

Russian forces pulled out of Chechnya after a devastating 1994-1996 war that left separatists in charge. In fall 1999, Putin sent troops back in after Chechen-based rebels attacked a neighboring region and after apartment-building bombings blamed on the militants killed about 300 people.