A teen boy found unconscious on the grounds of Santa Rosa's Montgomery High School was in a medically-induced coma Sunday due to the serious head trauma he suffered, police said.

But how the 15-year-old became injured remained a mystery, more than a day after a man walking his dogs found the boy, breathing but unconscious and bleeding profusely, on his back and slightly up against a wall outside the school's gym, Santa Rosa Police Sgt. John Cregan said.

Staff at Oakland Children's Hospital told police they expected the Maria Carrillo High School student to remain in a coma for three-to-five days.

"Then we will try to interview him," Cregan said.

Until then, police are left to follow several different investigative leads, which range from a fight to a fall from the schools' roof, Cregan said. No witnesses had come forward Sunday and police again asked for the public's help.

"There were people playing at the softball field at the time, it's an open campus," Cregan said. "We're hoping that anyone who saw kids who were playing on the roof or engaged in a fight will come forward."

Police originally reported the boy's age as 17, but officers later learned that he is actually 15 years old, Cregan said. Cregan said the Police Department would not be releasing the boy's name because police still don't know what happened to him, including if he was the victim of a violent crime.

Detectives Sunday were interviewing the boy's friends. Montgomery High School staff were reviewing surveillance footage from nearby cameras, Cregan said.

The teen lives with his family near Montgomery High's Hahman Drive campus, although he attends school across town in Rincon Valley.

The boy's father told police that his son left home to go for a walk at about 3:30 p.m. Saturday, 28 minutes before he was found unconscious. The boy's family told police it's not unusual for the teen to head outside on his own for a walk, Cregan said.

"From our understanding, he just went out for a walk, he wasn't upset, just going out for a walk, it's something he does as routine," Cregan said.

One theory police were investigating Sunday is that the boy became involved in a fight. His injuries included cuts and bruises on his arms and knuckles, Cregan said.

Another theory is that the boy fell from the school's roof.

Police said the boy had previously participated in "parkour," an outdoor sport that involves scaling, leaping and performing other acrobatics using everyday obstacles such as railings and walls as a course.

"He had done it in the past and one of his friends fell and broke a leg several months ago," Cregan said. "That is just one more investigative theory."

Anyone with information about the case can call the Santa Rosa Police Department's violent crimes team at 543-3590.

You can reach Staff Writer Julie Johnson at 521-5220 or julie.johnson@pressdemocrat.com. On Twitter @jjpressdem.