At least 56 people were hurt with six in intensive care as a result of the blast at the Zagorsk optical-mechanical plant in the city of Sergiev Posad, northeast of the Russian capital, the district administration said in a post on Telegram.
Videos of the blast geolocated by NBC News show a huge mushroom of smoke rising over the plant. One taken by nearby residents and posted to social media shows the moment a huge, fiery blast takes place behind a parking lot.
The incident happened around 10:40 a.m. local time (3:40 a.m. ET) in a pyrotechnics warehouse rented on the territory of the plant by a "private company," Moscow Region Governor Andrei Vorobyov said on Telegram.
Russia's federal investigative agency said it launched a criminal probe into the incident, citing a "violation of industrial safety requirements for hazardous production facilities," but said it was trying to establish the cause of the blast.
Emergency services were now going through the debris at the plant, which has been evacuated along with a nearby kindergarten, the governor said.
Russian state news agency Tass quoted the health ministry as saying three people had been rescued from under the rubble in serious condition.
People living in nearby houses had their windows shattered by the explosion, Vorobyov added, and a nearby sport complex was also partially damaged.
Vorobyov did not say what caused the blast, but Russian state media reported, citing emergency officials, that a “human factor” was behind the explosion. Local deputy Sergey Pakhomov also said “a violation of technological processes” was to blame, without elaborating.
Russian state media reported that the plant makes optical devices for law enforcement agencies and civilian use.
Early Wednesday, Moscow Mayor Sergei Sobyanin reported that two drones were shot down by air defense systems over the capital. He said there was no information on any casualties.
Vorobyov told reports at the site of the blast that investigators have so far not confirmed if a drone could have been involved in the incident.