An explosion near a restaurant at Kudrinskaya Square in Moscow killed three people on August 1.
The Interior Ministry said the blast occurred near Building 1 at about 8:10 p.m. local time.
Moscow police put the number of injured at 15.
The National Anti-Terrorism Committee gave a higher figure of at least 21 injured.
In its preliminary account, the committee said a woman suspected of carrying an improvised explosive device was trying to enter the restaurant when the device exploded.
The committee said the woman, a security guard and a customer were killed.
Authorities had not released the woman’s identity.
Officials had not established the cause of the blast or said whether it was intentional.
Police, fire and emergency teams responded, and access restrictions were imposed around the square.
The entrance to Barrikadnaya metro station was temporarily closed.
Russia’s central investigative authority began an investigation.