Four people were injured after a stun grenade went off during a school safety drill in the Moscow region, health officials said Tuesday.

Regional prosecutors said they were investigating the incident, which took place in the city of Shchyolkovo, located northeast of Moscow.

It was not immediately clear how or why a live stun grenade was being used in the drill.

The Interfax news agency, citing the regional health ministry, reported that three of the people injured were briefly hospitalized.

Officials did not specify whether those injured were students or teachers. Unverified reports suggested all four people were teachers.

In January, an accident involving a stun grenade killed one person and injured dozens of others at a police academy in the republic of Komi.

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