Coweta County coroner Richard Hawk said an autopsy began Tuesday after Hanson's death at the age of 29 on Monday night at Atlanta's Piedmont Hospital.
Hawk said Hanson was found "unconscious and unresponsive" Sunday in Coweta County, Georgia. He was initially taken to a hospital in Newnan before being transferred to Piedmont.
Hawk told The Associated Press on Tuesday night that "the cause and manner of death are still under investigation."
Sheriff's officials, meanwhile, describe the emergency call for Hanson before his death as an "overdose," but they don't provide other details.
Coweta County sheriff's officials described the Sunday emergency situation in a report released Wednesday. The report says Hanson wasn't breathing as rescue crews tried to help him at a friend's home south of Atlanta, in Newnan. They typed "overdose" in a small section titled "crime incident."
Two people -- a 40-year-old man and a 36-year-old woman -- were involved in the incident at the home, the report states. Neither they nor anyone else has been charged with any crimes.
Hanson, a 6-foot-6 right-hander, pitched for the Braves from 2009 to 2012 and won at least 10 games each season. He pitched for the Angels in 2013 before signing minor league contracts with the White Sox in 2014 and Giants in 2015.
Hanson was regarded as Atlanta's top pitching prospect when he made his debut in 2011. He posted an 11-4 record with a 2.89 ERA.
Hanson, a native of Tulsa, Oklahoma, was 49-35 with a 3.80 ERA in five major league seasons. He was 4-6 with a 4.76 ERA with two minor league teams in the Giants organization in 2015.
There were immediate expressions of shock and grief on social media after word of his death.
Former Braves teammate Chipper Jones said on his Twitter account: "My heart is broken today. Tommy Hanson was a great teammate, friend and pitcher. We all loved and pulled for him. We ALL will miss him. #RIP"
Braves president John Schuerholz said the team was "incredibly saddened" when it learned of Hanson's death.
"He was a favorite in the clubhouse and with our staff, and he will truly be missed by everyone in Braves Country," Schuerholz said in a statement. "Our thoughts and prayers are with his wife, family, former teammates and friends."
Former Braves closer Craig Kimbrel, now with the Padres, called Hanson "a great person."
Added Kimbrel on his Twitter account: "It hurts my heart to see him go so soon. My prayers and thoughts are with the Hanson family."
Hanson was the MVP of the 2008 Arizona Fall League. The league held a moment of silence for Hanson on Tuesday.
Information from The Associated Press was used in this report.