DALLAS -- Just days before Dallas Cowboys running back Ennis Haywood died of a brief illness, he had impressed teammates with his performance at workouts.

	Ennis Haywood was fifth on Iowa State's all-time rushing list with 2,862 yards.

An autopsy was to be scheduled Monday for Haywood, who was placed on life support at Medical Center of Arlington before his death on Sunday.

"The cause of death at this time is unknown,'' hospital spokeswoman Diane Stout told The Associated Press.

Stout said she doesn't know why Haywood, 23, was being treated. But she said an autopsy would determine the cause of death.

Haywood's sister-in-law, Angela Jernigan, said the player began vomiting in his sleep about 4 a.m. Saturday. She said after he stopped breathing about 6 a.m., family members called 911.

Haywood was placed on life support at the hospital, but died at 3:27 p.m. Sunday, Stout said.

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

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

"This is a time to offer support and our deepest sympathy to the family,'' Cowboys owner Jerry Jones said Sunday.

"Our hearts go out to them. This is a tragedy involving a person who was very well-liked and respected by anyone who had the opportunity to know him,'' said Jones, who spent much of Sunday at the hospital with Haywood's family.

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 1:30 rest between each run.

"He was blazing,'' veteran safety Darren Woodson said. "I was winded after the third one and he was talking and joking. We had a running joke between us. He always said he wasn't going to run me into the ground when we were running, but he always did.''

Haywood was expected to compete for a roster spot at halfback. He worked out four days per week at 7:30 a.m. with the first of four exercise groups.

Players received physical exams before the mini-camp began last week.

"It's such a shock because I talked to him every single day,'' said Cowboys offensive tackle Flozell Adams. "I just talked to him on Thursday and everything was fine.

"Every time I talked to him, he was never negative and always spirited. I know he was so excited about this season,'' said Adams. "He was in the weight room a lot and just getting ready. He kept saying this was going to be his year.''

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

At Iowa State, Haywood finished fifth on the club'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.

"Like everyone in the Cyclones' football family, we are shocked and saddened beyond words,'' Iowa State football coach Dan McCarney said Sunday. "Ennis was one of the classiest gentlemen there has ever been, on and off the field. He was an unbelievable competitor, and one of the most tenacious football players I've ever been around.''

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

"He always talked about going back to South Dallas and giving back to the community,'' said Woodson. "He was a South Dallas kid. That's where his heart was and that's who he wanted to help. When we walk in the locker room on Monday, you'll know he's not there. It's going to be tough.''