An official at the medical examiner’s office in Oklahoma City told ESPN.com that the cause of death would likely remain unknown until Friday at the earliest.

El Reno Police Chief Ken Brown said officers and medics responded to a call at a house in the town about 30 miles west of Oklahoma City about 9:25 a.m. concerning an unresponsive male “with unknown medical issues.” Brown identified the man as Box and said he first taken to an El Reno hospital, then transferred by air ambulance to Mercy Hospital in Oklahoma City.

Oklahoma defensive coordinator Brent Venables said linebacker Austin Box “exemplified everything you want in a player.” Venables, the first Oklahoma official to publicly answer questions, made a brief statement before answering questions for several minutes on Thursday evening.

“Every parent’s worst nightmare is to get that call,” he said. “We’re heartbroken.”

Venables said the team had all been notified of Box’s death.

“He was one of the most selfless guys I’ve ever been around, a great leader for us,” Venables said. “His greatest fear was to let down great coaches and great players… He wanted to live up to that in some way.”

According to the police report, El Reno police officer Todd Ward said that upon arriving at the house, he made contact with John Cobble III, who had identified himself to an emergency dispatcher as J.T. Cobble, who is the son of Tom Cobble, who was Box’s high school football coach in Enid.

Ward said in the report that Cobble III was performing CPR on Box and that “Cobble told me when I entered the room Box was in he believed he had overdosed.” On the police report, under the offense category, “controlled dangerous substance” is listed, and Ward checked the “drugs” box under a category listing possible or probable motivation.

Calls to a phone listing for J.T. Cobble in El Reno rang unanswered Thursday night.

During his conversation with an emergency dispatcher, the man who identified himself as J.T. Cobble told the dispatcher that Box was not breathing and that Box had been taking pain pills. The dispatcher walked J.T. Cobble through the CPR process. At one point, J.T. Cobble told the dispatcher, “I’m a little freaked out.”

“There’s a guy who stayed with me last night and he’s not responding to me,” Cobble said when asked what was happening. “He takes pain pills and he’s not responding to me.”

Asked whether Box was breathing, he said, “I don’t think so.”

Stacy Phillips, a spokeswoman for Mercy Hospital, said “out of respect for the family,” the hospital would not release information concerning Box. The family later issued a statement, saying “Austin loved everything about Oklahoma — the people, his hometown of Enid and his many close friends. Most of all, Austin loved his family and we loved him. We invite you to join us in celebrating his life.”

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

Oklahoma athletic director Joe Castiglione said Stoops is out of the country but that Stoops and the Sooners’ assistant coaches have been in contact with Box’s family.

“This is a devastating day for the OU family,” Castiglione said. “Austin was incredibly well-liked by his teammates, coaches and fellow students, and will be greatly missed by all of us.”

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

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.”

Former Oklahoma women’s basketball player Danielle Robinson tweeted, “Wow. Totally in shock. RIP Austin. Praying for the Box family and all of the Sooner football staff and players.”

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.

“He played 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 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.

“He stands for everything that’s right about this program,” Venables said. “He’s made a ton of big plays, and was instrumental in what we did to finish the way we did,” he said. “Without him, I’m not sure we would have finished the same way.”

Information from ESPN.com’s Big 12 blogger David Ubben and The Associated Press was used in this report.