All but two of the 118 hostages who died after Russian special forces raided a theater where they were being held captive succumbed to the troops' mysterious knockout gas, doctors said. However, the substance remained secret even as doctors treated the hundreds of survivors.

Three top Moscow doctors made the revelation Sunday amid public criticism over the number of innocent people killed at the theater and the way they died: at the hands of Russian authorities trying to save them.

Authorities did not tell medical officials what type of gas they pumped into the theater shortly before special forces troops raided it early Saturday, chief Moscow doctor Andrei Seltsovsky said.

That apparently left doctors unclear about how to treat the estimated 750 people inside.

Seltsovsky said doctors were familiar with the general category of the gas, which causes people to lose consciousness and can be used to anesthetize surgical patients, but were not told its name.

The gas can paralyze breathing, blood circulation, and cardiac and liver functioning, doctors said. The effects were worsened by the extreme conditions in which the hostages were confined for three days — little movement, lack of water, food and sleep, severe psychological stress — and by the chronic medical problems some suffered.

"In standard situations, the compound that was used on people 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 Interfax news agency, citing medical sources it did not identify, reported that 405 former hostages, including nine children, remained hospitalized Monday after 239 were released. On Sunday, doctors had said 646 people remained hospitalized, 45 of them in very serious condition.

Two foreign women, one Dutch and one Austrian, were known to have died, and officials in Kazakhstan said a 13-year-old girl from their country died — one of three children known to have perished.

There were about 800 people in the theater when it was seized by Chechen gunmen during Wednesday night's performance of the popular Russian musical "Nord-Ost," or "North-East."

Anguished relatives crowded the gates of Moscow hospitals, begging for news of their kin, while others scoured the city morgues.

Tatiana Lukashova's 26-year-old daughter, Masha Panova, was a hostage and now is missing.

Lukashova saw a broadcast on the ORT television station Saturday that showed her daughter lying on a mattress in a hospital corridor with an oxygen mask on.

"But we didn't hear what hospital it was, and our search through all the hospitals was in vain," Lukashova said in a telephone interview.

"It's unbelievable," she said, tears choking her voice. "Even the head of the district where we live went to meet officials of ORT to find out in which hospital they filmed the girl, but they told him they can't tell without permission from prosecutors."

She said she would visit all the Moscow morgues Monday "to at least exclude that she is there."

Even diplomats had trouble finding information about the estimated 70 foreign citizens who were among the hostages. U.S. consular officials searched the city's hospitals for one of two American citizens known to have been hostages.

President Vladimir Putin declared Monday a day of national mourning for the victims of the hostage crisis. Schools in Moscow were open Monday and started the day with a moment of silence, but many children's activities were canceled.

The death toll among the hostages stood at 118 on Sunday, including the 116 who died of the effects of the gas, a woman who was shot in the early hours of the crisis and a hostage killed by a gunshot wound to the head early Saturday.

Moscow officials said Monday that relatives of the dead would receive about $3,150 in compensation, while hostages who survived would get half that, Interfax reported. The city will pay for funerals, it said.

Officials said three gunmen were captured, and authorities searched the city for accomplices who may have escaped. The Federal Security Service said 50 assailants were killed at the theater, and several were shot in the head apparently as they lay incapacitated from the gas.

Some of the attackers who burst into the theater Wednesday night had explosives strapped to their bodies, and 18 were women who said they were widows of Chechens killed by Russian forces.

They mined the theater and threatened to blow it up unless Putin withdrew Russian troops from the rebellious, predominantly Muslim region of Chechnya.

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

In 1995 and 1996, rebels seized hundreds of hostages in two raids in southern Russia near Chechnya, and dozens of people died in both cases. Many of them were killed when Russian forces attacked the assailants.

Meanwhile, security remained tight in the capital and police arrested a Chechnya resident in downtown Moscow after finding an explosive substance on him and in his car, the ITAR-Tass news agency reported Monday. The man also had extremist Muslim literature, it said.

Police also said three Chechnya natives have been detained on suspicion of involvement in an Oct. 19 car-bomb blast outside a McDonald's restaurant in Moscow that killed one person, ITAR-Tass said. It was unclear whether that number included a suspect arrested last week.