A 44-year-old man and a 14-year-old boy died when an explosive device went off in a flat in Moscow.
A 12-year-old was also taken to hospital with serious injuries late on Saturday, according to the authorities.
The 14-year-old allegedly pulled out the fuse of a grenade, causing the explosion, media reported.
A woman present at the scene was reportedly unharmed.
The Central Investigation Committee in Moscow said that a criminal case for illegal possession of weapons had been opened after the detonation in the flat.
The authorities did not give any details about the explosive device itself.
The 14-year-old allegedly brought the "technical device" home.