Russia's emergencies ministry says the search of the concrete rubble from a partially collapsed apartment building has ended and the number of deaths stands at 39. The ministry declared the operation by hundreds of rescue workers completed Thursday, three-and-a-half days after a section of a 10-story apartment block collapsed in an explosion.

A 10-month-old boy pulled from the wreckage in the city of Magnitogorsk on Tuesday was the only person found alive in the debris. He was being treated in Moscow for serious injuries but was described as being in stable condition on Thursday.

The number of deaths stood at four for most of Monday, but climbed quickly after two nights of winter weather. Russian authorities acknowledged from the start that the extreme cold gave rescue crews less time to reach trapped survivors.

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Hope flared when the baby boy was extracted from the wreckage alive on Tuesday. He was flown to a Moscow children's hospital for treatment. The baby suffered frostbite from lying in the rubble for 35 hours -- temperatures were as low as minus 4 F.

Rescue workers clear the debris at the site of an apartment block collapse in Magnitogorsk, Chelyabinsk Region, Russia, after a Dec. 31, 2018 explosion. TASS/Getty

State news channel Rossiya-24 cited doctors saying the child also had injuries indicative of "crush syndrome" - major shock and kidney malfunction characteristic of people trapped under heavy objects. His condition was assessed as serious but stable, the Russia Health Ministry said Wednesday.

Another mysterious explosion

On Thursday, the mayor of Magnitogorsk told journalists there was no connection between the explosion at the apartment building and the fiery explosion of a minibus on the same street a day later.

The vehicle explosion took place about 1.5 miles from the apartment block. Police said the minibus was carrying gas canisters but have given few other details.

But video of the burning vehicle, including some posted on YouTube by Russia's RT satellite TV channel, featured sounds resembling gunfire and an observer's voice is heard saying people are shooting assault rifles.

Unconfirmed reports have suggested the apartment blast could have been a terrorist act and that the occupants of the exploded vehicle were suspects.

"I want to say that there are no clear facts showing any kind of confirmation of what is described on the internet today," Mayor Sergei Berdnikov said of the reports, according to state news agency Tass.