DENVER (Aug. 21, 2005) -- The cause of offensive lineman Thomas Herrion's death cannot be determined until toxicology tests are performed, a process that usually takes three to six weeks, a coroner said.

The 23-year-old offensive guard for the San Francisco 49ers collapsed in the locker room minutes after the team's exhibition game against the Broncos on a 65-degree evening in mile-high Denver. He was taken to the hospital and pronounced dead shortly after.

"We didn't see anything happen," 49ers defensive lineman Marques Douglas said. "I sat by my locker and prayed for him."

Howard Daniel, an investigator with the Denver coroner's office that performed an autopsy on Herrion, said nothing was readily apparent about why he died.

"There's no conclusion, pending further studies," Daniel said.

The death once again spotlights how dehydration and obesity affect athletes, especially the huge linemen who play in the NFL.

Herrion was 6-foot-3, 310 pounds -- fairly average for an NFL lineman, but considered obese within standards routinely accepted by the medical community.

"Our thoughts are with the Herrion family and the 49ers," NFL spokesman Greg Aiello said. "We have been in contact with the 49ers throughout the day to offer our assistance and to learn the details of what happened."

Herrion, who played in college at Utah, was on the field for San Francisco's 14-play, 91-yard drive that ended with a touchdown with 2 seconds left in the game. After the game, he was noticeably winded as he walked off the field, but didn't look much different than teammates who played beside him at game's end.

The death comes a little more than four years after offensive lineman Korey Stringer of the Minnesota Vikings died of heatstroke during a training camp practice when the heat index soared to 110.

NFL teams since have increased efforts to teach players about how to manage the heat. Temperatures at game time in Denver's thin air were in the mid-60s with 50 percent humidity, although experts say heatstroke can happen even in cool weather.

It is not known whether Denver's mile-high altitude could have contributed to the death. As a college player at Utah, Herrion played games at high elevation and would have been more used to those conditions than many.

In 1979, St. Louis Cardinals tight end J.V. Cain died of a heart attack during training camp. Chuck Hughes, a Detroit Lions wide receiver, died of a heart attack during a 1971 game in Detroit against the Chicago Bears. In April, Arena Football League player Al Lucas of the Los Angeles Avengers died of a spinal-cord injury after making a tackle.

Herrion, a first-year player with the 49ers, spent part of last season on the San Francisco and Dallas practice squads. He also played this season with the Hamburg Sea Devils of NFL Europe.

Herrion played in junior college at Kilgore College in East Texas. Travis Fox, the offensive coordinator at Kilgore, said he shared an apartment with Herrion for two weeks this summer. Herrion had returned to the school to get in shape before reporting to the 49ers.

Fox said Herrion never struggled during intense drills in 97-degree heat. He added that the lineman had no injuries or health problems while playing at Kilgore.

"The young man was in shape," he said.

Herrion's nicknames at Kilgore were "Train" and "Big T." Fox said he was called "Thunder" in Germany because his head was too big for a regular helmet.

Fox said Herrion always talked about his niece, and family was a big motivation for playing.

"When he got here," Fox recalled, "the first thing he told me was, 'I'm going to make this team and buy my mom a nice house."'