Several employees at a branch of Russia’s Alfa Bank in downtown Moscow were taken hostage by an attacker thought to be armed with an explosive device, local media report, citing law enforcement.

Moscow police responded earlier on Saturday to a high-risk call from one of the bank’s employees, who said the assailant had burst into the building and threatened to blow it up. At least five clerks are assumed to have been inside at the time.

Later reports said that only one person remains in the building and is being held hostage by the unknown perpetrator.

The Alfa Bank office, located on a busy avenue in central Moscow, was quickly sealed off by arriving police patrols, with officers establishing contact with the suspect.