Beverly Hills Shooting -- Actor's Bro Clings to Life

Mark Ruffalo's brother is clinging to life right now after he was shot in the head in Beverly Hills Monday.

Police sources tell TMZ Scott Ruffalo was the victim of a "suspicious shooting" that took place in his own Beverly Hills apartment.

We're also told cops are trying to determine if anything criminal went down -- meaning they haven't ruled out an accidental shooting or a suicide attempt.