HAYWARD — A university police chief allegedly stabbed his stepson repeatedly in the chest with a power drill and fired his gun inside a Hayward home on Monday during a bloody fight with the 20-year-old, authorities said Tuesday.

The Alameda County Sheriff’s Office said it believes Sonoma State University police Chief Nathan Johnson stabbed his stepson during a struggle, possibly puncturing his lung.

“Whether or not it was self-defense we don’t know,” sheriff’s Sgt. Ray Kelly said Tuesday. “It was a mutual combat situation. They both had varying accounts of what occurred.”

Johnson told authorities he fired his off-duty weapon into a wall inside the home as a warning shot, Kelly said. The altercation happened inside a family home in the 3300 block of Costa Drive in Hayward. Johnson called the sheriff’s office to report the fight at about 10:30 p.m. Monday.

Deputies later found the stepson, identified as Elijah Latimer, outside a nearby home with multiple stab wounds and a punctured lung, Kelly said.

Both Latimer and Johnson were taken to a hospital to treat their injuries. Latimer’s injuries are not considered life threatening. Johnson suffered minor injuries to his face, Kelly said.

No arrests have been made. The investigation was complicated by conflicting reports and the refusal of a family member who witnessed the incident to cooperate with authorities.

The sheriff’s office on Tuesday continued its investigation, which will be forwarded to the Alameda County District Attorney’s Office to decide if charges should be filed.

Johnson has served as Sonoma State’s police chief since 1999, according to the university’s website. In his more than 30 years in law enforcement, Johnson has received awards of valor from the California Attorney General’s Office and the California Police Chief’s Association, was recognized as a volunteer in New York City after the 9/11 terrorist attacks and has taught at the University of San Francisco, according to a university news release in 2010.

Calls to Johnson were not returned. University Affairs Vice President Don Condron said Tuesday afternoon that campus leaders were looking into the news report about Johnson, but that they had little information. Johnson was not on campus Tuesday, Condron said.

Staff writer Katy Murphy contributed to this report. David DeBolt covers Oakland. Contact him at 510-208-6453. Follow him at Twitter.com/daviddebolt.