Complex sentence: Before the Moscow theater hostage crisis, Barayev was believed to have been killed by Russian forces in Chechnya, as the Russian government claimed he had been killed by troops ten days prior to the beginning of the siege.
Split into simpler sentences: Before the Moscow theater hostage crisis, Barayev was believed to have been killed by Russian forces in Chechnya. Indeed, this is what the Russian government claimed to be the truth, that he had been killed by troops ten days prior to the beginning of the siege.