Seau was drafted into the NFL in the first round in 1990 out of the University of Southern California. He debuted with the San Diego Chargers, establishing his Hall of Fame potential as a Pro Bowl staple.
He left San Diego before the 2003 season to join the Miami Dolphins and spent parts of the last four seasons with the New England Patriots before retiring in late 2011. Seau amassed 1,526 tackles, 56.5 sacks and 18 interceptions in his 20-season career.
Seau's death rocked the Southern California city of San Diego, where he was a member of the 1994 Chargers -- the only team in franchise history to make it to the Super Bowl.
His reputation as a fierce tackler -- with a heart of gold for his community work -- earned him admiration and a legion of fans.
Through most of his career he avoided negative publicity that plagued some in the NFL.
But in October 2010, he made headlines when he was charged with domestic violence after an incident with his girlfriend, and then hours after his arrest he drove off a cliff and crashed on the beach. Investigators later ruled the crash an accident, saying he fell asleep at the wheel.
On Wednesday, Seau made headlines again. His girlfriend, who had just returned home from a workout, found him in the bedroom of his home with a gun lying next to him, authorities said.
It is not clear if Seau left a note or an explanation.
In the hours before his death, Mitchell said he sent text messages to his ex-wife and their three children.
"They said simply, 'I love you," Mitchell told CNN. "They actually responded back to him with 'Love you too, Dad.'"