A 19-year-old Vallejo resident is fighting for his life and an armed suspect is on the loose after a mid-day shooting aboard a San Francisco Municipal Transit Agency light rail vehicle Monday afternoon.

According to SFMTA spokesperson Paul Rose, the shooting was reported at 12:23 p.m. Monday on a southbound T-Third light-rail vehicle, near Third Street and Palou Avenue.

The Ex reports that two passengers were involved in a confrontation that "led to gunfire." The victim was transported to San Francisco General Hospital with life-threatening injuries, a police spokesperson says. According to KCBS, he was shot in the head.

The shooter fled on foot, KRON4 reports, and remains at large as of Tuesday morning. KCBS reports that "a few people were detained at the scene, but it was unclear if they were involved" in the shooting.

Rose says that the crime was caught on tape, as each Muni LRV has "about eight to 12 cameras per vehicle." According to a police spokesperson, investigators are reviewing the footage, and are working to determine a motive in the shooting.

According to police, the suspect is a man between 18-25 years old dressed in a hooded sweatshirt and baggy pants.

Police say that there were "numerous witnesses to the shooting," but are always happy to hear from more. They ask anyone with info on the crime to contact their Anonymous Tip Line at 415-575-4444 or to text a tip to Tip411 with SFPD at the beginning of the message.