|Moscow, Russia (CNN) -- An explosion late Friday at a nightclub in central Russia killed at least 101 people and injured at least 140 others, Russian officials said.
Eighty-five of the injured were in critical condition, officials with the Russian Emergency Situations Ministry.
Explosives experts of the Federal Security Service examined the club building and found no traces of explosives, the ministry said.
The preliminary theory behind the blast in the city of Perm, 900 miles east of Moscow, was "unsanctioned use of pyrotechnical devices," the ministry said.
"The breach of technology while setting off a firework was the cause of the accident," said Vladimir Markin, a spokesman for the Investigative Committee of the Russian Prosecutor's Office. "It was not a terrorist act," he told Russian news agencies.
Map: Blast blamed on fireworks
Burns and smoke inhalation caused the deaths, Markin said, adding that authorities have opened a criminal investigation.
Russian news media said the blast occurred as a party was in full swing at The Lame Horse nightclub, which was celebrating its eighth year in business.
Russian President Dmitry Medvedev sent a number of high government officials to Perm to handle the consequences of the accident, organize medical assistance to survivors and investigate the event.
Prime Minister Vladimir Putin dispatched from Moscow two planes equipped to carry people with extensive burns.
The explosion occurred a week after a luxury express train from Moscow to St. Petersburg derailed, killing 26 people and injuring than 100 others. Russia's Federal Security Service blamed the incident on an improvised explosive device that had been placed beneath the tracks by unknown bombers. It was the most lethal terrorist act outside of Russia's troubled North Caucasus region since 2004.