An unidentified gunman opened fire outside the Moscow headquarters of the Russian Federal Security Service (FSB) on Thursday night, killing a security officer and injuring five other people.

Few details of the incident have been released to the press as of Friday morning beyond an FSB statement that the gunman was “neutralized” at the scene and was not able to enter the headquarters building.

The shooting occurred within a few hours of Russian President Vladimir Putin’s annual press conference and occurred while Putin and a number of top officials were attending a concert commemorating “Security Services Day,” a national holiday celebrating the founding of Russia’s secret police, that was held less than a mile away. The FSB is the successor to the Soviet Union’s infamous KGB. Ironically, some bystanders who heard the gunshots assumed they were celebratory fireworks.

“Since the beginning of 2019, 54 crimes have been prevented, including 33 terrorist attacks,” Putin said during his speech at the event. “Today, the special services’ accurate, prompt and coordinated work has allowed achieving meaningful success in ensuring security across Russia.”

Euronews reported the attacker was armed with a “Kalashnikov assault rifle.” According to other news agency reports, one of the security officers protecting FSB headquarters returned fire and killed the assailant.

Some of those news reports indicated there were as many as three gunmen involved in the attack, two of them made it into the lobby, and all of them were killed by security forces. The FSB denied these accounts and insisted there was only one attacker. Smartphone video posted to social media by eyewitnesses showed several armed men running away from the building as gunshots rang out, but their identity is unclear. Witnesses thought at least some of them were traffic cops.

Russian state media on Thursday night referred to the attack as an “act of terrorism,” although this has apparently not been confirmed by the government.