FORT WORTH, Texas -- Dallas Cowboys running back Ennis Haywood died accidentally from a mixture of drugs and ethanol complicated by his asthma, the Tarrant County medical examiner's office has ruled.

"Although the mixed ethanol and drug ingestion was the primary cause of death, a significant condition contributing to the death was bronchial asthma," the office said in a statement issued Friday.

Haywood, 23, died during May minicamp at Medical Center of Arlington, where he was taken after he began vomiting in his sleep. Paramedics were called after family members were unable to find a pulse.

Further tissue and toxicology tests were conducted after an initial autopsy failed to show the cause of death.

The medical examiner's office said that it had determined he died from "mixed ethanol and drug ingestion (codeine and unidentified benzodiazepine)." The death was formally ruled accidental.

It also confirmed what his mother, Carol Haywood, said at the time -- that her son could have been a victim of asthma.

A woman who answered the phone at her home on Friday afternoon said Carol Haywood wasn't immediately available for comment.

The player's agent, Ronald Del Duca, did not immediately return a phone message left by The Associated Press.

Benzodiazepine is sometimes used to treat asthma. Ethanol is a form of alcohol used in a variety of ways, including in beer and whiskey. Codeine is an opiate used to treat pain.

Haywood was an undrafted free agent from Iowa State, where he finished fifth on the team's all-time rushing list with 2,862 yards and sixth in touchdowns with 27. As a senior, he rushed for 1,169 yards and 14 touchdowns to lead the Big 12 Conference.

The Dallas Carter High School graduate had been an honor student with a 3.5 grade-point average.

He was on the Cowboys' practice squad last season and led the team in rushing in the preseason with 120 yards on 31 carries.