A 15-year-old boy seriously injured at Montgomery High School over the weekend had been on the school's roof previously, possibly lending weight to one of several investigative theories as to how he was hurt, Santa Rosa police said Monday.

While detectives were still exploring the possibility that the boy was assaulted as well as other leads, they may not be able to determine what happened until they speak to the injured teen, Sgt. Dave Linscomb said.

"There's really no direct evidence to indicate he was assaulted," said Linscomb. "It's quite possible he fell from a roof. But just because we don't know at this time, we're still looking at any possible option."

The Maria Carrillo student was found at 4 p.m. Saturday, unconscious and bleeding from injuries to the back of his head, near the gymnasium of the Hahman Drive school.

The boy was on his back up against a wall, police said. He was flown to Children's Hospital Oakland, where he remained Monday, hospital staff said. Police said he was in a medically induced coma, having suffered major head trauma. He also had cuts and bruises on his arms and knuckles.

The boy's family told police he left home at about 3:30 p.m. to go for a walk, which was typical for the youth. A Santa Rosa doctor walking his dogs called 911 at 3:58 p.m. to report finding the injured and unconscious boy.

It wasn't clear Monday why the teen had been on the roof in the past. The sergeant said he didn't have detailed information on the prior incident. "We know he's been up on the roof before," Linscomb said.

A fall from the roof would have been more than 20 feet, police have said.

"We're not seeing anything on the surveillance footage. There is no surveillance that covers that area of the school, unfortunately," said Linscomb.

Friends also haven't been able to shed light on what happened.

Officers had learned during the initial investigation that the youth had participated in parkour. The outdoor sport involves scaling, leaping and performing other acrobatics and making a course from everyday obstacles such as railing and walls.