Sumter County coroner Verna Moore said Tuesday that an autopsy revealed Jackson likely had an enlarged heart for years.
The 26-year-old Jackson, who played for the Hamilton Tiger-Cats, was found unresponsive Monday at his girlfriend's home and pronounced dead at a hospital.
Jackson played safety and special teams for South Carolina from 2001-04. He signed with the New York Giants in 2006, and was allocated to NFL Europe and selected by the Berlin Thunder.
He signed with the Tennessee Titans in 2005 as an undrafted free agent, but was released during training camp. He subsequently joined the Montreal Alouettes' practice roster, and then signed with the Tiger-Cats in January.
Jackson had been living with his family in South Carolina and training at his alma mater, Sumter High School, before returning to Canada for the upcoming season. Cleo Jackson said her stepson had been in good health and had lifted weights on Saturday.
"We teased him all the time, 'You'd better stay in shape,'" she said, with a laugh. "No one you can talk to can say anything bad about him."
Jackson had 159 tackles and two quarterback sacks in 43 games for South Carolina and was named a Southeastern Conference Defensive Player of the Week.