Updated

Oklahoma linebacker Austin Box had five prescription painkillers and an anti-anxiety drug in his system when he died, according to a preliminary toxicology report released Tuesday by the state medical examiner's office.

The death of the 22-year-old Box two months ago was ruled accidental. In the report, the agency said the combination of drugs likely caused pulmonary edema, or fluid in the lungs, and aspiration pneumonia, which is an inflammation of the lungs caused by inhaling foreign substances.

"There is no greater pain than the loss of a child," Box's parents, Craig and Gail Box, said in a prepared statement. "The pain is intensified by knowing that the death of your child could have been prevented.

"Anyone that knew Austin would give testament to his pure heart. The love and pride we feel for our son cannot be diminished by the cause of his death. He gave us so much joy and so many wonderful memories. He will forever be 'Mommy's baby' and 'Daddy's little boy.'"

A friend found Box unresponsive in the friend's El Reno home and called authorities the morning of May 19. The friend told a dispatcher Box wasn't breathing, that Box had been taking pain pills, and later told a police officer "he believed he had overdosed."

An autopsy found the painkillers oxymorphone, morphine, hydrocodone, hydromorphone and oxycodone in Box's system, along with the anti-anxiety drug alprazolam. The report noted Box's significant medical conditions included cardiomegaly, or an enlarged heart, and a chronic pain history.

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.

But, as his family noted, he had a long history of injuries during the past seven years, many of which required surgery. The most recent came last August, when his parents said he had a disc rupture in his back and he lost the feeling in his left foot.

"We were certain his career was over," they said. "As always though, he battled back when he saw the team needed him."

The 6-foot-1, 228-pound Box returned to start the final 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. He was expected to start this season for the Sooners.

In an interview with The Oklahoman, attorney Craig Box recalled taking a four-day trip with his son to watch the St. Louis Cardinals play and that he never saw his son take any medicine other than "liquid Advil."

Gail Box, a school counselor, said after her son underwent elbow surgery in 2009 and was prescribed painkillers, he refused to take all the medicine.

"He only took that medication for a couple of days and then he said, 'No,'" she told the newspaper.

She described her son as a "silent sufferer."

In their statement, Box's parents said "it is with much sadness; we look back and see that recently Austin had turned to other methods of managing his pain." They said they're hopeful people using such a combination of painkillers "will see this tragic accident as a message and think about the consequences."

They said their son's greatest fear was letting others down.

"Our greatest regret is that Austin did not feel he could share his pain with those who loved him, and those he touched," they said. "He chose to suffer in silence rather than to feel he let someone down, or hurt his family."

In a statement, school President David L. Boren said the tragedy of Box's death "should not detract from our memories of his wonderful spirit, courage and generous concern for others."