San Francisco 49ers lineman Thomas Herrion, who died Saturday after a preseason game in Denver, had never tested positive for any banned substance since he entered the NFL two years ago, sources familiar with his league drug profile told ESPN's Chris Mortensen on Monday. The 23-year-old Herrion had tested negative on three occasions in the past 12 months for steroids and performance-enhancing drugs, including ephedra products. He also was clean in other league-administered tests for street drugs, the sources said. The Denver coroner's office performed an autopsy Sunday, but said no cause of death could be determined until toxicology tests were performed. The tests usually take about three to six weeks. "We lost a teammate and a very good friend," Niners coach Mike Nolan said on Sunday, as the shock of the loss sunk in. On Monday, less than 48 hours after their teammate collapsed and died, the 49ers went back to work.