The San Francisco 49ers somberly returned to work Monday, trudging through a brief afternoon practice after spending the morning attempting to sort through their still-raw emotions about young offensive lineman Thomas Herrion collapsing and dying following a preseason game Saturday night in Denver.

Questions remained about the cause of the 23-year-old guard's death. Herrion's agent said that the 49ers initially told him Saturday night that Herrion had suffered a massive heart attack. But the agent, Fred Lyles, said he wasn't taking that initial statement as the definitive word on the matter, and a 49ers spokesman said that anything said to Lyles that night was spoken in the heat of the moment and the team doesn't know what caused Herrion's death.

Members of the Denver coroner's office performed an autopsy Sunday but have not determined a cause of death, pending the results of toxicology tests.

"That's what they said to me that first night when they took him to the hospital, that it looked like he'd had a massive heart attack," Lyles said in a telephone interview. "I'm just waiting to see what the final answer is. They really don't know."

The coroner's office has indicated that it could take three to six weeks for the final toxicology results to come in. Lyles said he's hopeful that preliminary findings could be available in about a week.

"Hopefully we'll get some more answers," he said.

Herrion weighed about 310 pounds, and some of the offensive linemen who used to labor beside him admitted to being shaken by his death not only because they lost a friend, but also because it reinforced fears they might have about their own health.

"It's scary," guard Justin Smiley said. "It's deadly scary."

Wiemi Douoguih, the head of sports medicine at Washington Hospital Center, said it's reasonable to assume that Herrion's bulk contributed to his death.

"The guy was a big man," said Douoguih, the team physician for the Washington Nationals and a consulting physician for the Washington Redskins and Baltimore Ravens. "It's been demonstrated that players who are overweight are at risk for heart problems. It may not be the definitive answer, but his weight was probably a significant factor. They [the 49ers] had a long drive at the end of the game. He was on the field for a long time. But you won't know until you get the final report."

Herrion participated in a 14-play touchdown drive near the end of the game, then collapsed in the locker room as the 49ers were finishing a team prayer.

Elliot Pellman, the chairman of the New York Jets' medical department and the NFL's medical liaison, said it was an "open question" whether Herrion's death was related to his weight, and declined to give an opinion on the possible causes.

Some medical experts say that heart defects are the leading cause of sudden death in young athletes, and it's possible that an electrical problem in a structurally normal heart might not have been revealed by an autopsy. But one expert in the field, speaking on the condition of anonymity because he did not want to offer an opinion publicly on a case in which he is not involved, said that a sizable heart attack would have shown up in an autopsy and the coroner normally would report a heart attack if the autopsy showed that there had been one.

The 49ers spent about 50 minutes on the field in their first practice since Herrion's death. "I think it's therapeutic to get back out here on the field for the players and coaches alike," Coach Mike Nolan said.

But tackle Jonas Jennings said: "It was a waste for me mentally. I wasn't there, but you have to be a professional. It's life you're thinking about now. I felt like he was on my watch . . . and I kind of took it personally . . . . Growing up in the projects, I saw people shot. I saw people stabbed. But there was always a reason behind it. This was a man who was just finishing the Lord's Prayer."

The 49ers began their day with a roughly three-hour meeting in which Nolan had a team physician, the team chaplain and an outside grief counselor address the players. Nolan said he was particularly concerned about his players because they watched Herrion collapse and struggle for life.

"It was painful," tackle Kwame Harris said. "It was difficult. I never went through anything like that before. It makes you appreciate life."

Said center Jeremy Newberry: "We lost a great person. It makes you appreciate life and the health of my family and my kids. We're pretty fortunate to be able to do this."

The 49ers have scheduled a private memorial service for Tuesday. NFL Players Association Chief Gene Upshaw is scheduled to be on hand for that. Herrion's funeral is scheduled for Saturday in Fort Worth, his home town. Nolan, 49ers co-owners John York and Denise DeBartolo York and NFL Commissioner Paul Tagliabue are scheduled to attend.

Nolan said he spoke to several fellow NFL head coaches -- including the Arizona Cardinals' Dennis Green, who was the coach in Minnesota when Vikings tackle Korey Stringer died from a heatstroke suffered on the training-camp practice field four years ago -- and had messages from others. Nolan said he briefly thought about canceling Friday night's exhibition game against the Tennessee Titans at home but decided against it. The team will return to its normal work schedule Wednesday, he said.

Staff writers Leonard Shapiro in Washington and T.R. Reid in Denver contributed to this report.

Practice "was a waste for me mentally," tackle Jonas Jennings said. "I wasn't there, but you have to be a professional." Scott Peters (67), Tony Wragge work through their grief.Thomas Herrion, 23, was 6-3, 310 pounds. "The guy was a big man," said Wiemi Douoguih of Washington Hospital Center.