Law enforcement sources tell TMZ cops plan to make an arrest in the death of Mark Ruffalo's brother Scott next month.

Our sources would not say who will be arrested but said the person or persons will be charged with murder.

Scott Ruffalo died from a bullet wound to the head earlier this month in his Beverly Hills apartment. The two people in the room claim Scott was playing Russian Roulette. The L.A. County Coroner begs to differ, ruling the death a homicide.