A factory that manufactures precision optical devices for the Russian military near Moscow has exploded, leaving at least 52 people injured.

The Zagorsk Optical-Mechanical Plant, located in Sergiev Posad, has been producing essential equipment for the armed forces since World War II. It currently makes precision optical devices, including night vision goggles and binoculars, as well as medical equipment. The factory is owned by Shvabe Holding, which itself is a part of Rostec, Russia’s large state-owned defence conglomerate.

According to Russian officials, the explosion occurred due to a technological failure in a fireworks warehouse “being rented by a well-known local company.” Russian independent media outlet Verstka has named that company as Piro-Ross, which recently filed for bankruptcy, and is thought to have had government contracts with the Russian Ministry of Defence. Russia’s Investigate Committee has opened a criminal case on “violation of industrial safety requirements for hazardous production facilities.”

The director of Piro-Ross has rejected the official story, and has stated that the explosion came from a different warehouse that stored metal pipes.

Videos shared on social media show a huge mushroom cloud following the explosion, and debris flying in all directions damaging property.

This explosion follows a number of other blasts at Russian industrial sites, some of which have been linked to Ukrainian drone attacks or pro-Ukrainian saboteurs.

Writing on social media, Moscow Region Governor Andrey Vorobyov revealed that 38 buildings were affected, including two schools.

“Rescuers will need about 12 hours to remove the rubble. There may still be people there. Three have already been extracted. Their condition has been assessed by doctors as serious. Nearly 160 rescuers and law enforcement agencies are working at the site. The exact causes of the explosion are [still] to be found out,” he said. “A total of 56 people sought medical help. 30 are now in hospital. Six of them are in intensive care. Another 26 people went to the trauma centre but they did not need hospitalisation. After examination, doctors let them go home.”

Following initial speculation, the Russian emergency services also emphasised the explosion did not occur due to a drone strike.