(CNN) -- A medical examiner was working Friday to determine what killed an Oklahoma University linebacker found unconscious a day earlier on the floor of a home, police said.
Austin Box, 22, was declared dead Thursday morning at an Oklahoma City hospital, police told CNN affiliate KWTV.
Paramedics responded to a call that Box was unconscious and not breathing at a home in El Reno, according to a 911 tape released by the affiliate.
"He takes pain pills, and he's not responding to me," a voice on the tape told a police dispatcher.
Box returned to the starting lineup during the midseason after missing the first part of the year with a back injury.
He was considered a big part of the university's late-season resurgence, according to the coaching staff, and was slated to begin his senior year as a starting middle linebacker.
News of his death rocked the university. Grief counseling was being made to his teammates and close friends, said Joe Castiglione, Oklahoma athletic director.
"We're all shocked and heartbroken," Bob Stoops, the head football coach, said in a statement on the university's website.
Box was "an integral part in our success the last three years and was looking forward to a big senior year," Stoops said. "As heart-wrenching as this is for us, we know it's even more difficult for his family. More than anything, our thoughts and prayers are with them."
Box's family issued a statement, thanking the university and others for their support.
"Most of all, Austin loved his family and we loved him," the statement said.