The perpetrator of the Metro Red Line attack has been identified as 36-year-old Ephraim Okorie.
Police say Okorie became violent after they escorted him off a train for possessing drug paraphernalia at a station in East Hollywood.
The injured sergeant and Okorie were both taken to the hospital.
It remains unclear if medical personnel were able to reattach the sergeant's finger.
Okorie is now booked for the crime of "mayhem," with his bail set at $100,000.