Rescuers, pausing frequently to put their ears to the rubble to listen for signs of life, pulled scores of corpses out of a mass of concrete planks and metal beams Thursday after the snow-laden roof of a Moscow market collapsed.

At least 56 people were killed and 32 injured, said Emergency Situations Ministry spokeswoman Natalya Lukash.

Virtually all the victims were believed to be workers from Azerbaijan and other former Soviet republics, among the thousands who have poured into the Russian capital to fill low-paying jobs such as those at the city's produce and housewares markets.

The probable cause was either the buildup of heavy snow, design flaws or maintenance errors, Moscow Prosecutor Anatoly Zuyev said. Prosecutors have opened a criminal investigation on charges of negligence leading to deaths, the RIA-Novosti news agency reported.

One of the survivors, Ukhtai Salmanov, a 52-year-old herb-seller from Azerbaijan, said he left the market just before the roof gave way around 5 a.m.

"I heard a loud noise and I fell to the ground and lost consciousness. When I came to, I was lying by the entrance.

"There was smoke and people were screaming," Salmanov said, his clothes covered in dust.

Fighting back tears, he said his three sisters, who also worked in the market, were killed. There was no way he could have saved anyone, he said, because a mound of rubble blocked him from reaching them.

Dogs were helping to try to locate survivors and bright searchlights illuminated the rubble after darkness fell.

Cries and shouts rang out from the crowd of relatives gathered near the site as emergency workers read off the names of the hospitalized. One woman was pulled away, wailing, after hearing her brother was killed.

"I have a cousin there. I've been calling him since morning but at first there was no answer, and now the phone does not ring," said Eshkin Mekhvaliyev, a young Azerbaijani man.

Medical workers tried to help a man trapped under a slab of concrete that left only his hand visible, giving him painkillers through an intravenous drip.

Machines were brought in to blow warm air into the rubble to try to keep victims alive in the near freezing temperatures.

Trapped survivors called relatives using mobile phones, helping rescuers find them, said Yuri Akimov, deputy head of the Moscow department of the Emergency Situations Ministry. But hours passed in the afternoon with no rescues.

In mid-afternoon, emergency workers had to shift their efforts to fighting a fire that broke out on the edge of the building, sending acrid smoke billowing into the air. The fire was started by a spark from an electric saw that ignited some paint, said Emergency Situations Ministry spokesman Viktor Beltsov. He said it posed no threat to anyone who might have been trapped.

Rescue workers used metal cutters, hydraulic lifters and pickaxes to clear the ruins of the Basmanny market and knelt down to shout into the holes in search of survivors.

Emergency officials said it was impossible to say how many people had been in the market at the time of the collapse, but survivors and witnesses said there could have been up to 100 people or more. The market is one of the city's biggest wholesale and retail markets.

Two to three inches of wet snow had fallen overnight, on top of 18.5 inches that had fallen since the start of winter, the Russian Weather Service said.

Mayor Yuri Luzhkov, who went to the site to oversee rescue efforts, said the roof, which had a special gutter, was designed to clear itself of snow and there was no particular need for workers to clear snow from the roof.

The market was built in 1974, Luzhkov said. The Interfax news agency reported the market was designed by Nodar Kancheli, the same architect who drafted the plans for Moscow's Transvaal water park, where the roof collapsed in February 2004, killing 28 people. Prosecutors have blamed that collapse on design flaws.

Kancheli came to the market after the collapse and was later questioned by investigators, Russian news agencies reported.

"I think one possibility is a big buildup of snow," Kancheli told Ekho Moskvy radio. "And they set up kiosks on the mezzanine, which was not originally planned."

He said corrosion also could have played a role.