Middlesex prosecutors say they are investigating the shooting of a 22-year-old man by a Marlborough police officer Monday night. Bryce Coutinho was killed in the confrontation, which happened after police were called to a domestic disturbance at a residence on Meadowbrook Road. Police responded to the residence at approximately 8 p.m. Monday. They were called to assist in the removal of an unwanted person from the residence, Middlesex District Attorney Gerard T. Leone Jr.’s office said. Police were confronted at the home by a man armed with a knife. A Marlborough officer fired his gun, Leone's office said. Coutinho was pronounced dead at the scene.