CBS News Correspondent Bill Gasperini reports that the blast occurred at around midnight local time, as hundreds of people slept in their apartments inside the large residential complex.
Authorities said more than 100 people were rescued, and 60 were hospitalized with serious injuries after being pulled from the wreckage. Up to 100 people could be buried under the mounds of smoking debris, they said.
Prime Minister Vladimir Putin said the blast was apparently caused by a natural gas explosion Â– many apartments in Moscow are old and havenÂ't been well-maintained in recent years.
But many eyewitnesses did not smell gas before or after the blast. Some officials said it might have been a bomb.
Â"The nature of the damage and the number of casualtiesÂ" suggests an explosive device was placed in the building, the Federal Security Service, the country's main intelligence agency, said in a statement.
Â"It does not look like gas,Â" added Sergei Shoigu, head of the Emergency Situations Ministry.
Moscow has been on edge since a bomb exploded at a downtown shopping mall last week, killing one and injuring dozens. Last weekend, a huge explosion killed scores at an apartment complex in the restive Dagestan region, where Russian troops are fighting Islamic militants.
An anonymous caller told the Interfax news agency that the Moscow explosion and the weekend blast were in response to Russia's military campaign. There was no way to determine the authenticity of the claim.
The Moscow blast completely collapsed all nine stories in the center section of the block-long building, but it left apartments standing on either side.
Echo of Moscow, a radio station, said authorities were searching for a car that had been seen shortly before the explosion. The report did not say why the car was being sought.
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