He died at Self Regional Hospital after going into cardiac arrest about an hour before at his family's home in Greenwood, said Marcia Kelley-Clark, chief deputy coroner for Greenwood County.
An autopsy conducted today showed that Adams had an enlarged heart, a condition Kelley-Clark said can often lead to a heart attack. But Adams' relatives didn't know about it.
"Nobody was aware of any kind of medical condition," Kelley-Clark said.
Toxicology tests are being run by the State Law Enforcement Division, though drug use was not suspected as a factor in Adams' death. Those results probably will not be available for at least two months, Kelley-Clark said.
Adams was selected fourth overall in the 2007 NFL draft by Tampa Bay but was traded to the Chicago Bears in October of last year for a second-round pick in the 2010 draft.
The 6-foot-5, 258-pound defensive end was well-known among Clemson fans for breaking up Wake Forest's field goal try and returning it for a touchdown in 2006 to defeat the Demon Deacons.
Tommy Bowden, who was Adams' head coach at Clemson, said he couldn't believe the young player was gone.
"I just couldn't believe it was Gaines," Bowden said. "I will always remember the smile he had on his face, and I will always remember his patience."
With the Bears, Adams played brief stints on defense. He made five tackles.
"We thought the player was a quality player, and if we get him in our system with our coaches, we have a lot of confidence we can make it work," Chicago general manager Jerry Angelo said after the Bears acquired Adams.
Bears linebacker Brian Urlacher told the Chicago Tribune that Adams' death was "crazy."
"I didn't know him that well because he came in during the middle of the season," Urlacher said. "But I did know him. I still saw him every day when I went into work. It's just weird.
"I had a teammate die when I was in college. You just don't know how to handle it. It's just sad, man. It's a bad deal."