Dallas Cowboy and former ISU running back Ennis Haywood died of unknown causes Sunday in Dallas. Haywood was 23 years old.

According to Associated Press reports, Haywood began vomiting in his sleep early Saturday morning. Family members called paramedics, who transported Haywood to the Medical Center of Arlington.

At the hospital, Haywood remained on life support long enough for friends and family members to visit on Saturday and Sunday. Haywood died at 3:27 p.m. Sunday, hospital spokeswoman Diane Stout said.

Haywood rushed for 2,862 yards at Iowa State between 1998 and 2001, including 1,237 in Iowa State’s 9-3 season in 2000, the ninth highest single-season rushing total in school history. Haywood also ranks fourth all-time at Iowa State in all-purpose yardage, totaling 3,468 yards.

Only Troy Davis has gained more yards per carry as a Cyclone than Haywood, averaging 5.6 yards per carry to Haywood’s 5.0.

In both of his seasons as the starting tailback, Haywood was a first-team all-Big 12 selection.

Haywood signed with the Cowboys as an undrafted free agent following the 2002 NFL draft. According to the Dallas Cowboys Web site, Haywood led the team in rushing during the 2002 preseason with 121 yards, before spending the regular season on the Cowboys’ practice squad.

This preseason, Haywood was expected to compete for a spot on the Cowboys’ 53-man regular season roster.

Haywood is survived by his wife, Crystal, and a three-year-old daughter. Crystal Haywood is expecting the couple’s second child in the next month.

An autopsy performed Monday yielded no conclusive results, said Linda Anderson, public information officer for the Tarrant County Medical Examiner’s Office.

“The doctor didn’t see anything unusual,” she said.

Anderson said toxicology and tissue tests should shed some light on the cause of Haywood’s death. The tests will show if any potentially harmful substance was in Haywood’s body at the time of death, she said.

While it usually takes six to eight weeks for the tests to be performed, Anderson said Haywood’s results may come back more quickly.

“We’ve put a priority on [this case],” Anderson said.

The ISU athletic department is collecting messages from fans, friends and family to collect in a book for Haywood’s family.

Comments can be left through the athletics Web site at www.cyclones.com or sent to Ennis Haywood Book, c/o Iowa State Athletic Media Relations, Jacobson Athletic Building, 1800 S. Fourth St., Ames, IA 50011.

— The Associated Press contributed to this story.