The L.A. County D.A.'s Office will review battery allegations against Justin Bieber ... and multiple law enforcement sources tell us ... the case will be taken "seriously."
We're told when the L.A. County Sheriff's Dept. concludes its investigation ... the case WILL be referred to the D.A. for review and possible prosecution.
TMZ broke the story ... one of Bieber's neighbors claims the singer was tearing through their Calabasas community Tuesday at speeds approaching 100 mph, and when the neighbor confronted Bieber he allegedly spit in the guy's face and threatened to kill him.
Several prosecutors in the D.A.'s Office tell TMZ ... spitting crimes are taken seriously for 2 reasons -- first, it's considered an extremely aggressive and "disgusting act," and second, it poses potentially serious health concerns.
We're told Bieber's neighbor wants the singer prosecuted and will fully cooperate with authorities.