La Mesa, CA (KSWB) — A boy apparently shot his sister while playing with a rifle in their La Mesa home Monday, police said.

The 12-year-old boy was playing with an AR-15 rifle that accidentally discharged, shooting his 17-year-old sister on the couch, La Mesa police Lt. Matt Nicholass said.

La Mesa police were informed of the shooting around 9:45 a.m. at the 8800 block of Jefferson Avenue. No adults were home at the time of the shooting.

The teen was transported to a local hospital to be treated for gunshot wound to her left upper shoulder, according to the lieutenant.

Her condition was unknown.

"It's a tragic event where a gun got into a child's hands and someone was injured as a result," Nicholass said.

Investigators are determining who owned the weapon and if any charges were going to be filed.