Holmes was charged a few weeks ago in a Dairy Queen robbery, which may have been a factor leading to the shooting, Garrett said. But he said the teen's death remained under investigation.
He said there had been no indication that Holmes might be suicidal.
Chelsea Smith, a senior, said she was seated in the gym bleachers when Holmes appeared at the top of the bleachers and walked down to the center of the gym court.
"He had a gun to his head," she said.
Smith said a coach walked toward Holmes, who pointed the gun at the coach until the coach retreated. She said Holmes then fired into the ceiling and shot himself.
Grief counselors were sent to the school, said Nancy Pierce, a spokeswoman for the Mobile County school system. She said classes would resume Friday.
Traffic was heavy outside the school around noon as concerned parents arrived. Police were directing traffic and the departure of students was orderly.