University of Oklahoma football player Austin Box died from an accidental mix of powerful drugs, according to the Oklahoma Medical Examiner's office.

A toxicology report performed on Box, who died May 19 after collapsing at a friend's house, revealed Box had five separate pain-killers and an anti-anxiety drug in his system at the time the 22-year-old football player died.

The toxicology report said the University of Oklahoma player had the following drugs in his system: oxymorphone, morphine, hydrocodone, hyrdomorphone and oxycodone and the anti-anxiety drug alprazolam in his system.

The Oklahoma Medical Examiner's office said the probable cause of Box's death was pulmonary edema and aspiration pneumonia that likely was the result of mixed drug toxicity.

Box would have been a senior this season at the University of Oklahoma. He suffered multiple injuries in his playing career, and had elbow surgery in 2009 and back trouble during the 2010 season.

The parents of Austin Box released two statements when the Oklahoma Medical Examiner's office report was made public. They read:

There is no greater pain than the loss of a child, the statement from his parents said. The pain is intensified by knowing that the death of your child could have been prevented.

It is with much sadness; we look back and see that recently Austin had turned to other methods of managing his pain. Methods that we hope if others are employing, they will see this tragic accident as a message and think about the consequences. Our greatest regret is that Austin did not feel he could share his pain with those who loved him, and those he touched. He chose to suffer in silence rather than to feel he let someone down, or hurt his family.

His parents were reportedly shocked at the findings, according to a close family friend who spoke with KWTV.

(Box) was always the biggest, strongest, and the toughest, Wade Burleson said. Great expectations upon him. So, if there was a problem, obviously he suffered silently.

Burleson said Box was looking toward the upcoming University of Oklahoma football season, and what would have been the player's senior year.

Box was found unconscious in an El Reno, Oklahoma home in May. He was taken to a hospital, where he was pronounced dead. A 911 call to police from J.T. Cobble, a friend of Austin Box, told police, He takes pain pills, and he's not responding to me.

On the 911 call, Cobble could be heard calling out Box's name to try and get his attention, but his friend, the University of Oklahoma player, was unresponsive.

Box was staying at the El Reno house with Cobble, who is the son of Tom Cobble, Box's former high school football coach in Enid, Oklahoma. According to a police report, Cobble performed CPR on his friend to try and keep him alive.