The nation's top investigative agency, the Investigative Committee, said an explosion triggered by a gas leak apparently caused the pre-dawn collapse in the industrial city of Magnitogorsk. The New Year's Eve accident shocked Russians and marred the mood on the nation's most beloved holiday.
Authorities said five people were hospitalized with injuries and 35 others remained unaccounted for. Russian officials acknowledged that the odds of finding anyone alive in the debris looked increasingly slim given the extreme weather.
Nearly 1,400 rescue workers searched in temperatures of -17 degrees Celsius (1.4 degrees Fahrenheit). The overnight forecast called for temperatures to plunge to -24 degrees Celsius (-17 F) overnight.
"The chances are reducing with time," Health Minister Veronika Skvortsova responded curtly when pressed by reporters on the likelihood of finding trapped survivors. "But incredible stories do happen."
Emergency officials deployed powerful heaters to raise temperatures in the wreckage in case anyone trapped there was at risk of dying of hypothermia.
"We must work as quickly as we can as temperatures don't give us any time to linger," Deputy Emergency Minister Pavel Baryshev said during a conference call with local officials.
Officials said they had all the necessary equipment for a thorough search, but the work was proceeding slowly due to fear other sections of the 10-story building might collapse. Residents of some sections were evacuated as a precaution.
The structure was constructed from concrete panels. One that was left hanging in the collapsed section of the building came crashing down as television stations aired live broadcasts from the scene.
Russian President Vladimir Putin visited the accident site about 1,400 kilometers (870 miles) southeast of Moscow and went to a local hospital where injured residents were taken.
At the hospital, Putin spoke to a 13-year old boy, who sustained head injuries and frostbite after spending an hour under the rubble before being rescued.
"You will get well soon, you are a fighter," Putin told the boy after hearing that he was practicing martial arts.
Another survivor, Yulia Gavrilova, recalled the harrowing moments of the collapse.
"We were sleeping and I woke up feeling I was falling down," she said. "I first thought I was dreaming it. Then I woke up for real and realized that I was standing outside, the wall was not there any longer. My mother was screaming that she couldn't breathe and my son was screaming from another corner."
Local authorities initially said 68 people registered as residents in the collapsed section of the building were missing, but later tracked down some of them.
The Russian Emergency Ministry later said that 35 people remained unaccounted for, and it wasn't clear if they were in the building when it collapsed.
The Local governor, Boris Dubrovsky, told Putin that authorities published the missing persons list in the hope they were somewhere else when the explosion happened and would report their whereabouts. He promised to quickly provide new apartments for those who were left homeless.
Gas explosions in Russian homes and businesses are common, and they are usually blamed on neglect of safety rules or poor maintenance.