"Preliminary information is that his death stems from a medical issue, and our investigators currently have no reason to believe otherwise," police in the Atlanta suburb said in a statement.
Police told family members they found Thomas in a shower at his home.
"He had been suffering from seizures for over a year, and we believe he had a seizure when he was showering," LaTonya Bonseigneur, a first cousin who grew up with Thomas and was so close they considered themselves siblings, told the AP early Friday.
"We're not sure when he died. We just spoke with him yesterday."
She added, "He was alone and a friend couldn't get hold of him, so he called his driver, who has a key because of these seizures, and he went into his home and found him in the shower."
Several Broncos players tweeted emotional responses to the news.
Patrick Smyth, the Denver Broncos chief communications officer, released a statement from former Broncos Super Bowl champion Peyton Manning that said in part "D.T. was a better person than he was a player, and he was a Hall of Fame player. That tells you how good of a person he was."
Thomas, 33, was drafted by the Broncos in the first round of the 2010 draft, leaving the NFL as the Broncos' second-leading receiver in franchise history. He was selected to five Pro Bowls, won two AFC Championships, and Super Bowl 50. The Broncos celebrated Thomas' career during the team's regular-season home opener against the New York Jets, the last team Thomas played for, in September at Empower Field at Mile High.
"Demaryius had an incredible NFL career and was such a big part of everything we accomplished during his many years as a Bronco," said John Elway, Broncos President of Football Operations, after Thomas announced his retirement. "D.T." was the complete package as a wide receiver, growing into one of the very best at his position. The combination of his size, speed, strength, and athleticism was unmatched. Demaryius' remarkable consistency and production were instrumental in our offense setting historic records and our team winning a lot of games, including two AFC Championships and Super Bowl 50."
Thomas finished his career third in the franchise in receptions (718), second in receiving yards (9,814 yards), first in 100-yard receiving games (36), and second in receiving touchdowns (66). From 2012-15, he had 90 receptions and 1,300 receiving yards each season, joining Pro Football Hall of Famers Marvin Harrison and Jerry Rice as the only players to reach those marks in four consecutive seasons. He was also the third Bronco to make five consecutive Pro Bowls.
Two plays ring in the minds of Broncos fans when looking at Thomas's career. The first was in 2011 when Tim Tebow connected with Thomas on an 80-yard touchdown pass to beat the Pittsburgh Steelers 29-23 in overtime of the AFC Wild Card Game. The second was catching Peyton Manning's 509th career touchdown pass in 2014, surpassing Brett Farve to set the all-time passing touchdown record.
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