OKLAHOMA CITY -- Oklahoma linebacker Austin Box died Thursday, and an investigation is ongoing into what might have led to his death, authorities said.

The Norman university sent out statements from President David Boren and athletic director Joe Castiglione confirming Box's death. Cherokee Ballard, a spokeswoman for the state Medical Examiner's office, said Box's body was being sent there for examination.

"The University family is deeply saddened by the tragic death of student-athlete Austin Box," Boren said. "Our hearts go out to his family and friends."

El Reno Police Chief Ken Brown said officers and medics on Thursday morning responded to a call at an address in the city about 30 miles west of Oklahoma City of an unresponsive male "with unknown medical issues." Brown identified the man as Box and said he first was taken to an El Reno hospital, then transferred by air ambulance to an Oklahoma City facility.

Brown said authorities are investigating "as to the cause" of Box's condition. Stacy Phillips, a spokeswoman for Mercy Hospital in Oklahoma City, said "out of respect for the family," the hospital would not release information concerning Box.

Box, a 6-foot-1, 228-pound senior from Enid, started the last five games last season for the Sooners, recording his second career interception in a win against Oklahoma State and making eight tackles as Oklahoma beat Connecticut in the Fiesta Bowl.

Castiglione said Oklahoma coach Bob Stoops is out of the country but that Stoops and the Sooners' assistant coaches have been in contact with Box's family.

"Austin Box will be greatly missed by his many friends and teammates in the Athletics Department and all across campus," Castiglione said.

He said grief counseling will be made available to members of the team and to Box's close friends.

"We're all shocked and heartbroken," Stoops said in a statement released by the school. "Austin was a great young man, a great young man to coach and a great teammate. He played an integral part in our success the last three years and was looking forward to a big senior year. 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."

Some of his teammates responded quickly on Twitter after learning of Box's death. Quarterback Blake Bell said "the sooner family will miss you brother! You are in a better place." Defensive end Frank Alexander tweeted: "We gone hold it down for u homie."

Box starred at Enid High School, leading the Plainsmen to the Class 6A title game in 2006, playing quarterback, running back, wide receiver and free safety at different times.

He redshirted with the Sooners in 2007 and played in 10 games as a freshman in 2008, starting four at middle linebacker. He suffered a knee injury late in the season and missed the Big 12 Conference championship game. He played in 10 games in 2009, starting a game each at outside linebacker and inside linebacker.

Box missed the first five games of his junior season with a back injury suffered before the season began but ended up being a key contributor to a team that finished 12-2. He was expected to vie for a starting spot as a senior.