FORT WORTH, Texas -- Further tests will be conducted to determine what killed Dallas Cowboys running back Ennis Haywood after an autopsy Monday failed to show a cause of death, a medical examiner's office spokeswoman said.

"At this point, we don't have a cause of death," said Linda Anderson, Tarrant County Medical Examiner's Office spokeswoman. The office is awaiting results of toxicology and tissue tests, which normally take six to eight weeks, she said.

Haywood died Sunday at Medical Center of Arlington, where he was taken early Saturday morning after he began vomiting in his sleep. Paramedics were called after family members were unable to find a pulse, said Haywood's mother, Carol.

The 23-year-old was placed on life support, but died at 3:27 p.m. Sunday, hospital spokeswoman Diane Stout said.

His family chose to discontinue life support Sunday, after several friends, colleagues and family members were able to come to the hospital and say goodbye to Haywood, the Fort Worth Star-Telegram reported Monday.

"It's tough, but I'm OK," Carol Haywood said.

She said her son could have been a victim of asthma, but more medical information was needed.

"He was so well thought of by his teammates and, in my personal time with him, he lifted me up with his personality," Cowboys owner Jerry Jones said after leaving the intensive care unit Sunday. "It's just a tragedy."

Haywood, an undrafted free agent from Iowa State who participated in the Cowboys' three-day mini-camp, completed four 300-yard sprints on Friday with 90 seconds' rest between each run. He was expected to compete for a roster spot at halfback.

His wife, Crystal, is expecting the couple's second child in a couple of weeks.