MOSCOW (Oct. 30) - The gas used to end the Moscow theater siege Saturday was based on the powerful opiate Fentanyl, Russian Health Minister Yuri Shevchenko said Wednesday, ending a four-day mystery.

Special forces pumped the highly addictive painkiller into the theater, where more than 800 hostages were being held by 50 Chechen separatists, before storming it early Saturday.

Authorities said they were forced to use the gas to knock out the rebels, who had been demanding Russian troops quit their southern homeland. The rebels had threatened to blow up the theater should the security forces storm it.

Russia had come under heavy international pressure to identify the active agent in the gas, which was responsible for the deaths of all but two of the 119 hostages who died in the siege. Most died of respiratory and heart failure.

''To neutralize the terrorists, a substance based on Fentanyl derivatives was used,'' Shevchenko said in comments broadcast on Russian television.

''On their own, these substances cannot lead to a fatal outcome,'' he said.

But Shevchenko added that in this case the anesthetic was given to people who already were in bad shape due to the conditions under which they had been kept as hostages. He denied earlier speculation that the special forces had used chemicals in the gas, possibly BZ, a nerve agent developed during the Cold War.

''I officially declare: chemical substances which might have fallen under the jurisdiction of the international convention on banning chemical weapons were not used during the special operation,'' he added.

Foreign Ministry spokesman Alexander Yakovenko said the use of the gas was ''fully in accordance with Russia's international obligations and the convention on chemical weapons and Russian law.''

A POTENT-OPIUM BASED NARCOTIC

Fentanyl is a potent-opium based narcotic that works on the brain's pain receptors. The drug works quickly and is used both for general anesthesia and light sedation, as well as in treating cancer patients.

However, it is also highly addictive and, like morphine, is sold as a street drug. While it is one of the safer drugs used in anesthetics it can, if taken in high enough doses, cause respiratory difficulties and death.

U.S. ambassador Alexander Vershbow said Tuesday hostages could have been saved if the doctors treating them had known what the gas was.

''It's clear that with perhaps a little more information, at least a few more of the hostages may have survived,'' he said.

The initial death toll was put at just 10, but rose steadily in the days after the storming, as doctors complained that they had not been told how to treat the sick.

Shevchenko said doctors were warned in advance of the operation, flatly contradicting what Moscow's top doctor and anesthesiologist had earlier told reporters.

''Specialists were warned, including me, even though the operation was an emergency,'' Shevchenko said, adding that thousands of doses of antidote had been prepared.

But Andrei Seltsovsky, chairman of the health committee of the city of Moscow, said Sunday he had received a call alerting him that there was an emergency only minutes before the first gassed hostages were taken from the theater.

Moscow's top anesthesiologist, Yevgeny Yevdokimov, told the same Sunday briefing that doctors could not give specific antidotes as they did not know what gas they were dealing with.

REUTERS  Reut13:57 10-30-02