MOSCOW, 28 October — All of the 116 hostages who died as a result of the operation to free them at a Moscow theater were killed by the effects of gas that special forces spread through the hall before storming it, Moscow’s head city doctor said yesterday. The gas, which has not been publicly identified, was aimed at knocking out the more than 50 assailants who had stormed the theater Wednesday night and held hundreds hostage until the pre-dawn raid by special forces on Saturday.

Head city doctor Andrei Seltsovsky said 116 people died of the gas and that another was killed by the gunmen in shooting that sparked special forces to mount their raid to free the hostages. One other woman was killed by the gunmen in the early hours of the crisis.

The fatal effect of the gas apparently was magnified by the weakened condition of the hostages, who had spent 58 hours under enormous stress and with little food or water, anesthesiologist Yevgeny Yevdokimov said, according to the Interfax news agency.

As of last night, 646 of the released hostages remained hospitalized Seltsovsky said. One hundred and fifty of them were in intensive care and 45 "in a grave condition". Dozens of freed hostages were released from city hospitals yesterday afternoon.

The unidentified chemical was so powerful that the Chechen fighters had no time to detonate the explosives strapped to their waists.

Sergei, 36, who declined to give his family name, said after he was released from hospital that the gas had smelled slightly bitter. Chemical warfare experts say nerve gas often smells of bitter almonds.

London-based security expert, Michael Yardley, said he believed the gas used was BZ, a colorless, odorless incapacitant with hallucinogenic properties, first used by the United States in Vietnam.

He said the symptoms displayed by the hostages in Moscow — inability to walk, memory loss, fainting, heartbeat irregularities, sickness — all pointed to BZ. According to the US Army, the side effects last 60 hours, Yardley said.

"The Russians wouldn’t want a big shout about it because it (BZ) is just the sort of stuff they are not supposed to have," he said. "It’s not specifically banned, but...it is in a sort of gray area."

A US Embassy spokesman said that Washington had asked the Russian authorities to reveal what gas was used by the special forces. "We asked the Russian authorities about the type of gas used but we have not been informed. We’re still waiting for a response," the spokesman, who asked not to be named, said.

"We have a rescued hostage in the hospital and it’s relevant to her treatment," the official said, adding that US consular officials were talking to her doctors and US physicians were on the way to the hospital.

Film taken after the special forces stormed the theater to free more than 750 hostages showed a woman slumped back on a chair with her mouth wide open and a bag of explosives tied to her front.

"A panic went up among us and people were screaming, ‘Gas! gas!’ and, yes, there was shooting," theater director Georgy Vasilyev, one of the hostages, said.

"But then everyone fell down quickly. And then I was told by one woman while we were in hospital together, but who didn’t fall asleep immediately because she covered her mouth and nose, that it was very strange to look at everyone.

"You see, when the shooting began, they (the rebels) told us to lean forward in the theater seats and cover our heads behind the seats. But then everyone fell asleep. And they (the rebels) were sitting there with their heads thrown back and their mouths wide open."

One hostage told Interfax that he saw the guerrillas convulse and slump because of gas. "After the first shots at the hostages gas came in, I saw how a terrorist sitting at the scene jumped up and tried to get a respirator. I saw how he convulsed and tried to put the mask to his face and then fell," the unidentified witness said.

Among the dead were four foreign hostages. They were Natalja Zjirov, 38, a Dutch national of Russian origin; Alexandra Litiaga, a 13-year-old Kazakh girl; 55-year-old Lyudmila Bogacheva of Belarus and an Austrian woman.

Authorities had done little to ease the anxiety of hostage relatives after the raid, giving little information on hostage identities, what hospital they were taken to or what condition they were in.

Hundreds of hostage relatives crowded outside hospital gates hoping for clues. Some saw their loved ones waving to them from hospital windows, but the hospitals prohibited visits.

Irina Ramtsova waited outside the black iron gates of City Clinical Hospital No. 13 with pictures of her father, Fyodor, a trumpet player at the theater. "We keep calling and calling and there is no information," she said.

Like Ramtsova, many were unsure where their relatives were, and came because the bulk of the survivors needing medical attention — some 320 — were at that hospital. "They are hostages again," one of the relatives shouted to the armed guards behind the gate.