At least three people were killed and 21 others injured after a homemade bomb exploded outside an upscale restaurant in central Moscow on Saturday evening, Russian authorities said.

According to officials, the blast occurred shortly before 8:00 p.m. local time near the Balzi Rossi Italian restaurant, located in one of Moscow’s iconic Stalin-era skyscrapers at Kudrinskaya Square.

Russia’s Anti-Terrorism Committee said an unidentified woman attempted to enter the restaurant but was stopped by a security guard. Moments later, an explosion occurred, killing the woman, the security guard, and a restaurant customer.

Authorities said those injured sustained varying degrees of injuries and were taken for medical treatment.

Investigators have not disclosed the identities of the victims or announced who may have been responsible for the attack.

According to Russian newspaper, Kommersant, citing unnamed sources, investigators believe the explosive device may have been intended to target guests gathered on the restaurant’s outdoor summer terrace.

The report also suggested that the bomb may have been detonated remotely and that the woman carrying the device might not have known it contained explosives.

Meanwhile, the restaurant’s official website indicated that the venue had been closed to the public at the time of the explosion because it was hosting a private event.