Moscow ? A mysterious knockout gas killed 116 hostages after Russian special forces stormed a Moscow theater to free them from Chechen terrorists, casting a shadow on what was first seen as a triumphant rescue operation. More than 150 additional hostages were in critical condition Sunday, the chief Moscow city doctor said.

The physician in charge of the city’s poison unit 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,” said Yevgeny Luzhnikov, head of the city health service Department of Severe Poisoning. The substance was described as akin to compounds used in surgical anesthesia.

The gas affected hearts and lungs, said Andrei Seltsovsky, the chief city physician. 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 Sunday.

The approximately 800 hostages were taken Wednesday night when an estimated 50 Chechen rebels stormed the theater during a popular musical. They demanded that Russia end its war in Chechnya.

The few dozen hostages who were well enough to be released Sunday could provide few clues as to the nature of the gas.

“We knew something serious was going to happen” when the gas started seeping into the hot auditorium that reeked of excrement, Mark Podlesny said as he walked out of Veterans Hospital No. 1 near the theater.

“I lost consciousness. Yes, there was a strange smell,” said Roma Shmakov, a 12-year-old actor in “Nord-Ost,” the musical in progress when the gunmen burst in.

A girl places flowers Sunday outside a theater in downtown Moscow. On Saturday, Russian special forces ended a hostage crisis at the theater with gunfire and sleeping gas that killed at least 116 people.

Outside hospitals where the hostages were taken for treatment, friends and family crowded the gates in futile efforts to learn if loved ones were inside. Authorities gave out little information on identities, conditions or where victims had been taken.

Even diplomats had trouble finding information about the estimated 70 foreign citizens who were among the captives. U.S. consular officials searched the city’s hospitals for one of the two Americans known to have been in the theater. A second American was found recuperating in a city clinic. At least two other foreigners one Dutch and one Austrian died.

Only on Sunday afternoon, more than 24 hours after the hostages were freed, did hospitals post 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 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.

Those who stopped to talk gave accounts that sometimes contradicted the official version.

A police officer, left, speaks with relatives of freed hostages attempting to enter the City Clinical Hospital No.13, where dozens of ex-hostages were treated after the Moscow theater hostage crisis. Hundreds of freed captives were taken to area hospitals, most of them suffering from sleeping gas that had been sprayed through the building before the raid.

Podlesny 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.

Podlesny and Georgy Vasilyev, producer of Nord-Ost, disputed Russian officials’ statement that the guerrillas had begun shooting hostages before dawn and prompting the special forces’ assault.

A total of 118 hostages where 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 Putin visited the special forces troops Sunday to congratulate them on the mission and declared today a national day of mourning. As troops that had surrounded the theater building began to withdraw, Muscovites placed flowers at the perimeter.

Many of the 50 assailants killed in the rescue mission died after being shot in the head, apparently while unconscious from the gas. The Federal Security Service said three other gunmen were captured, and authorities searched the city for accomplices or gunmen who may have escaped.

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.