MOSCOW — The roof of an indoor water park collapsed Saturday evening onto a crowded pool, killing at least 21 people and injuring about 106, Russian authorities said.
Some swimmers remained trapped in the rubble early today as rescuers raced to free them. With outdoor temperatures close to zero degrees, equipment was brought in to blow hot air into the heavily damaged building to help keep victims alive.
Moscow Mayor Yuri M. Luzhkov told reporters that the sharp contrast in temperature between the recreational area and the frigid outdoors might have triggered the collapse. The dome was apparently made of concrete, steel and glass.
"There is credible evidence that it was not a terrorist act," Luzhkov said.
"The collapse of the roof was recorded by video cameras.... The glass dome might have snapped because of temperature differences."
The Russian capital has been particularly on edge since an apparent terrorist bombing on the Moscow subway killed 41 people early this month. Some witnesses said Saturday that they had heard an "explosion," which authorities believed was the sound of the dome beginning to cave in.
There were 362 people in the pool and water entertainment area when the dome collapsed, and nearly 800 people in the facility, authorities said.
Terrified swimmers rushed from the building into the cold night air, some still in their swimsuits, as clouds of mist rose from the ruins of the heated pool. At least three children were reported among the dead, and 28 children were among the injured.
Authorities immediately opened a criminal investigation.
"It's for experts to decide what happened, whether it was a construction fault or whether there were faults in the use of the building," Anatoly Zuyev, Moscow's chief prosecutor, told reporters.
Luzhkov said late Saturday evening that there were "several niches" where people had been heard calling for help. Rescuers periodically called for moments of silence to listen for voices from the rubble. Two cranes lifted away pieces of concrete and metal, while rescuers cut through metal beams with blowtorches.
Ambulances rushed the injured to hospitals, while heavy equipment was brought in to clear snow from approaches to the building. Crews using fire ladders climbed to examine the dome, from which an additional fragment fell around midnight, apparently without causing additional injuries.
About 1 a.m. today, rescue efforts were concentrated on four sites in the rubble where voices had been heard, an emergency official told the Russian news agency Itar-Tass.
About 800 rescue workers were on the scene, including firefighters, construction workers and search-dog handlers, the agency said.
Built two years ago, the facility, which can accommodate up to 2,000 visitors, is one of several aquatic parks in the capital. It has three pools, including one for children, an artificial river and a water slide.
Russian television showed a jumble of collapsed wall or roofing materials that fell onto the twisted water slide, with a short palm tree sticking out from the rubble.
Ilya Lezhava, vice principal of the Moscow Architecture Institute, told Echo of Moscow radio that construction mistakes might well have caused the collapse. "Some section of the roof could have warped under the weight of snow or simply rusted," he said.
Given the harshness of Moscow's winters, "it is very important to correctly calculate the snow load on the roof and choose the construction material," he added.
Although early reports blamed a buildup of snow on the roof as the cause of the collapse, Luzhkov said he did not believe that that was the reason.