The knock-out gas that special forces pumped into Moscow's Dubrovka theater to end the hostage crisis five years ago sent baffled scientists scrambling in their laboratories in the United States and Europe.

Now, five years later, the verdict is in. The mysterious substance appears to have been an FSB-made version of carfentanyl, an artificial, opium-like substance that is 10,000 times more potent than morphine and usually used to immobilize large animals. And, as it turns out, the gas wasn't really a gas at all but an aerosol -- tiny particles that float in the air.

Scientists said that using the narcotic to knock out the 800 hostages and their 42 Chechen captors rather than risk a bloodbath was a wise decision, given the hopelessness of the situation.

"This was quite a cunning feat," said Thomas Zilker, a toxicology professor at Munich's Technical University who examined two German survivors after the attack.

He said the rescue operation after the release of the aerosol was probably to blame for most of the 129 hostage deaths. "Had they prepared themselves better for the medical aftermath, more lives could have been saved," he said.

Doctors who treated the hostages have said they worked in the dark without knowing what substance had been released in the theater to end the 56-hour siege in the early morning of Oct. 26, 2002. Government officials, who initially described the substance as a gas, still treat its contents as a state secret.

But a first clue about its composition came shortly after the end of the crisis when then-Health Minister Yury Shevchenko said it was a derivative of fentanyl, an artificial opioid about 80 times more powerful than morphine. One of fentanyl's most potent derivatives is carfentanyl, which is so powerful that a tiny drop can put down an elephant.

Russian and Western scientists who have examined former hostages said their findings point to carfentanyl as the mysterious substance. Lev Fyodorov, a former Soviet chemical weapons scientist who heads the Council for Chemical Security, an environmental group, said it was probably the Federal Security Service-developed narcotic more generally known by the code name Kolokol 1, or Bell 1.

Zilker, the German toxicologist, said two fentanyl derivatives were found in the urine of the German survivors he examined. He said the findings were actually made in the United States because fentanyl metabolizes quickly and the traces had already been too faint for any European laboratory to detect.

Zilker said he could not reveal the names of the derivatives. "I had to promise [the U.S. authorities] not to publish the results," he said in a telephone interview.

He said earlier reports that his team had found traces of halothane, a widely used inhaled anesthetic, were erroneous because of a glitch in his laboratory. "Some of our test tubes had contained traces of halothane from earlier use," he said.

Paul Wax, a top U.S. toxicologist, said the U.S. government had decided to keep its findings classified. But he agreed that carfentanyl was the most likely answer. "It is very intriguing because it possesses the ideal properties," he said by telephone from Paradise Valley, Arizona.

He called the agent ideal because it floats in the air and requires only a miniscule amount to get quick results. The Chechen hostage takers fell asleep so fast that they had no time to fire their weapons or detonate their bombs.

Spraying an aerosol is complicated, because it does not spread evenly like a gas. But the special forces were aided by the layout of the theater hall.

"They just used the ventilation system, which was very strong because the hall was very big," Fyodorov said in an interview in his apartment in southern Moscow. The agent, he said, originated from a generator placed in a space between the hall and the building's roof.

Emergency response workers picking up unconscious hostages at the theater were ordered to inject them with naloxone, a widely used opioid antagonist. All the hostages brought to at least one city hospital responded to naloxone.

But for many hostages, help came too late. Doctors had not been told what agent the hostages had inhaled, leaving them guessing how to treat their patients. Also, the unconscious people were rushed to hospitals in ordinary buses, many of them placed in seats or on the floor.

Fyodorov pointed out that when the Alfa special forces stormed the theater, they focused first on shooting dead the unconscious captors instead of helping the hostages. "There were no paramedics, no emergency ministry officials in the room," he said. "This was a case of gigantic unprofessionalism."

He said the use of a chemical agent was "utterly outside international and national law."

No officials were charged after the rescue operation, and an investigation by the Prosecutor General's Office was suspended after a year. The prosecutor's office declined immediate comment about the case.

The Kremlin has long maintained that the rescue effort was handled properly.

"All special forces involved in the operation acted in strict accordance with Russian and international law," Kremlin spokesman Dmitry Peskov said Monday.

He said the special forces officers had been "ready to die to save the hostages."

Fyodorov suggested that authorities used the agent again in October 2005, when they crushed an insurgency by Islamic militants in Nalchik, the capital of Kabardino-Balkaria. An Associated Press reporter noted at the time that troops fired gas grenades that caused at least one hostage to lose consciousness as they began storming a building where the insurgents were holding hostages. Doctors later said the hostages were suffering from the effects of an unspecified, nonlethal gas.

Peskov said he had never heard allegations that a chemical agent was used in Nalchik.

While morally dubious, the handling of the Dubrovka crisis could probably be justified under the Chemical Weapons Convention, which Moscow signed in 1997, said Jan van Aken, the Hamburg-based director of the Sunshine Project, an international nongovernmental organization against the use of chemical and biological weapons. The convention explicitly allows the use of chemical weapons for law enforcement, including domestic riot control purposes.

"They just need to argue that the siege was a domestic crisis and no act of war," van Aken said.

After the siege, the government sent a letter to the Organization for the Prohibition of Chemical Weapons explaining its actions, the organization's spokesman, Peter Kaiser, said via telephone from The Hague. He said he was not authorized to discuss the contents of the letter.

Van Aken called Dubrovka a prime example of how nonlethal chemical weapons should be avoided in both warfare and anti-terrorist operations.

He said people react differently to temporarily incapacitating agents like fentanyl and that the agents needed to be delivered in individual doses.

"It is in the nature of biochemistry that some will always suffer fatal results. Even under the best clinical conditions, an anesthesiologist's patients sometimes die," van Aken said.

"With the use of hand grenades in the same situation, the mortality rate would statistically not have been higher," he added.

With 129 fatalities out of 800 people, the Dubrovka death rate was about 16 percent. In comparison, a grenade attack typically results in 10 percent fatalities, according to a report published in the World Journal of Surgery in 1992.

A 2003 study by U.S. scientists Lynn Klotz, Martin Furmanski and Mark Wheelis found that even the most effective incapacitating agent could be expected to result in 10 percent fatalities. "Genuinely nonlethal chemical weapons are beyond the reach of current science," the study said.