In an evening news conference, Health Minister Yuri Shevchenko acknowledged that the gas was based on "derivatives" of fentanyl, a commonly used anesthetic, which he said was deployed to "neutralize the terrorists." Despite the large number of fatalities, he denied that doctors were ill-prepared to handle the consequences for the hundreds of hostages in the theater, and stressed that Russia's use of the gas did not violate the international treaty on chemical weapons, of which Russia is a signatory.

As a grieving city laid to rest at least 43 victims in funerals today, Danish authorities in Copenhagen arrested a top aide to Chechen separatist leader Aslan Maskhadov, saying he may have been involved in the Moscow hostage crisis and other terrorist attacks. The aide, Akhmed Zakayev, was detained late Tuesday, Danish authorities said, after attending the final session of the World Chechen Congress being held in the Danish capital -- a gathering that drew outrage from Russia for being held right after the Moscow theater siege. A Danish judge ordered Zakayev held until Nov. 12, pending investigation.

According to U.S. experts, fentanyl belongs to a group of medicines called narcotic analgesics that suppress breathing. A normal dose goes to the brain but then is quickly redistributed to the rest of the body, making it a short-acting anesthetic. But a larger dose is not redistributed as easily, remaining concentrated in the brain and shutting down respiratory functions. The drug naloxone counteracts its effects, but would have to have been administered by Russian rescue workers almost immediately.