The explosion, which occurred in the town of Pavshino at 9:10 a.m. Moscow time (6:10 a.m. GMT), completely ruined the three-storeyed brick plant building and damaged residential houses in the radius of more than 500 meters (1,640 feet). Water supplies to 150,000 people in the nearby area were disrupted.
"A high gas content inside the plant could have triggered an explosion," said Vladimir Stepanov, head of the regional emergencies center. "Experts are yet to determine the source of the leak."
Regional prosecutors have suggested methane blew up in the plant.
"Preliminary findings suggest it was a methane explosion. Repairs were done in the area the day before," prosecutors said.
But a fire security chief, Pavel Soldatov, said a gas leak was hardly the cause of the explosion.
"I was at the scene 15 minutes after the explosion and I am positive there was no smell of gas in the air," he said.
He also dismissed the depressurization of underground tanks at a gasoline station, built near the water-supply plant several weeks ago, as another version. He said gas station personnel would have noticed a leak.
A plant mechanic, who was inside the plant when the explosion occurred, also said there was no unusual smell inside the building. He said the explosion took place, when personnel were pouring off water from a collector drain, which needed repairs.
"When the explosion went off, I was near the pumps. We were pouring off water from the collector drain, which needed welding," Vladimir Rotar said in intensive care, where he is being treated for severe burns.
Rotar, who regained consciousness and got out of the debris on his own, also said all plant systems had worked normally.
Emergencies workers have been clearing the debris throughout the night.
Authorities said they had taken measures to prevent sewage spills into the Moskva River located 150 meters (492 feet) from the scene.
Three of the five sewage pumps have been inspected. One of them was less severely damaged and could be activated. Access to the other two pumps is still impossible, according to emergencies officials.
Authorities said cold water would be supplied to households twice a day, from 1 to 2 p.m. and from 9 to 10 p.m., and promised to restore normal supplies Monday. But schools and kindergartens will be closed Monday for sanitary activities.
Drinking water has so far been supplied to the region in trucks.
The explosion damaged 19 residential houses in the region, shattering windows and doors. Roofs and walls were seriously damaged in five buildings. Authorities have inspected the buildings and started repairs.
