MOSCOW (AP) _ The concave, snow-covered roof of a Moscow market collapsed early Thursday, killing at least 21 people and trapping about 10 people who were ``knocking and crying out,'' emergency officials said.

Rescue workers used metal cutters and pickaxes to break through the wreckage, kneeling to call into holes in search of survivors. Every few minutes, the rescuers turned off their electric generators and stood silently to listen for signs of life. About six hours after the collapse, rescuers had located just two survivors.

More people under the rubble were alive, Emergency Situations Minister Sergei Shoigu told reporters at the scene. ``They're knocking and crying out,'' he said.

Moscow city prosecutor Anatoly Zuyev said up to 40 people could have been in the Bauman market in eastern Moscow at the time of the early morning collapse, Russian news agency RIA-Novosti reported.

``We are working at full strength all over the territory of the market,'' said Yuri Akimov, deputy head of the Moscow department of the Emergency Situations Ministry. ``Rescue workers are working underneath the building to save those still alive, and we are hoping to save the majority of them.''

The head of the Emergency Situations Ministry, Yuri Akimov, told state-controlled Channel One television that survivors were using mobile phones to help rescuers find them. Rescuers were also using sniffer dogs.

Investigators were looking at three possible causes of the collapse: improper maintenance of the 30-year-old building, a buildup of heavy snow and errors in the building's design, the Interfax news agency quoted Zuyev as saying. A wet snow had fallen for much of the day Wednesday and overnight.

Moscow Mayor Yuri Luzhkov, who went to the site to oversee rescue efforts, ruled out terrorism as a possible cause, RIA-Novosti reported. Prosecutors have opened an investigation.

Ministry spokesman Viktor Beltsov said at least 21 people were killed and 24 injured in the collapse. A duty officer at the Emergency Situations Ministry, who spoke on customary condition of anonymity, said the death toll could have been higher had the collapse occurred while the market was open for retail business.

Ukhtai Salmanov, a 52-year-old herb-seller from Azerbaijan, said he had been heading out of the market at about 4:50 a.m. when he heard a loud noise and fell to the ground near one of the exits.

``When I came to, I was lying by the entrance. There was smoke and people were screaming,'' Ukhtai said, fighting back tears.

He said his three sisters, who also worked in the market, were killed.

Russian media said the entire roof, covering an area of about 2,400 square yards, had fallen onto the market stalls. Most of the victims were believed to be municipal and market workers, though Ekho Moskvy radio said there could also have been wholesale buyers in the building.

The collapse occurred on Defenders of the Fatherland Day, a Russian state holiday honoring the armed forces.

The Interfax news agency said the market had been designed in the 1970s 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 visited the market early Thursday.

``I think one possibility is a big buildup of snow,'' he 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.