UPDATE: The medical examiner's office in DeKalb County, Georgia confirmed to E! News that Paul Duncan's death is under investigation.
The football world has lost one of its own.
Paul Duncan, who briefly joined the Denver Broncos as an undrafted free agent, passed away July 16 at the age of 35. His wife, Ellen Duncan, confirmed his death in an Instagram post later that day.
"Yesterday, Friday July 15th, Paul went into cardiac arrest while on a run in our neighborhood," she wrote. "Today he was pronounced brain dead."
Ellen continued, "We will have a medical examination to understand cause of death. His body will be donated to people in need of organs and to medical research." She concluded her message by thanking everyone for their "prayers and support."
Paul leaves behind his two daughters, Dillon, 6, and Sloane, 4. According to his obituary in The Atlanta Journal-Constitution, he was a proud girl dad.
"He embraced the role of 'girl dad,' from building playgrounds in the backyard to doing hair before school," the obit read. "Dillon and Sloane loved Paul's impromptu dance parties in the kitchen and his adventures with them in the water, whether it was at the lake with family or at the pool with friends."
Before Paul joined the NFL, he was an offensive lineman for the Notre Dame Fighting Irish football team from 2005 to 2009.
The Notre Dame football team mourned his death in a tweet: "Our prayers go out to the family and friends of Paul Duncan. A great teammate, but more importantly a loving husband and father."
—Additional reporting by Spencer Lubitz
(Originally published on July 22 at 2:42 p.m. PT)