At least 60 people were injured and eight remain unaccounted for following an explosion at an industrial plant northeast of Moscow on Wednesday, Russian state news agency TASS reported quoting a local official

The blast took place at Zagorsk Optical and Mechanical Plant, in the town of Sergiev Posad, TASS reported. The plant is a developer and manufacturer for optical and optoelectronic devices for the Russian military, law enforcement agencies, industry and healthcare.

In the hours following Wednesday morning’s explosion, Russian state media reported that the source of the detonation was a pyrotechnics warehouse rented by a third company on the site of the Zagorsk Optical and Mechanical Plant.

Rescue teams continued to work at the site on Wednesday afternoon amid concerns that more people could remain under the rubble. “Rescuers will need about 12 hours to clear the rubble. There may still be people left, three have already been removed – doctors assess their condition as serious,” Andrei Vorobyov, governor of the Moscow region, said.

In an interview with Russian state media outlet RIA Novosti, the governor said: “One thing is clear, that the explosion was powerful, and it was in the pyrotechnics workshop.”

An eyewitness named Nikolai said in an interview with MASH, an independent Russian media blog present at the scene, that there used to be a munition factory at the site before “it was sold and turned into pyrotechnics factory.”

“The company went bankrupt; it is not clear who worked there and what they did there. What caused the explosion is also unclear,” Nikolai said.

Russia’s Investigative Committee has opened a criminal investigation into the incident.

A representative from the Investigative Committee told TASS news agency there was no evidence that the plant had been hit by a drone, and that the cause of the blast was being investigated. Ukraine has launched a series of drones at Moscow in recent weeks.

“As for the explosion, it was very powerful,” they said. “And now the operational services are engaged, investigating. I cannot comment on what it was, because journalists traditionally ask the question: Was it a blow from above or an explosion inside – this is done by specialists.”

Windows in several houses nearby were shattered by the blast wave, TASS added.

Sergiev Posad is part of what’s called the Golden Ring – a cluster of ancient towns with Russian orthodox churches around Moscow. The city’s main cathedral and surrounding buildings are on the UNESCO World Heritage List.