Seau, who played in the NFL for parts of 20 seasons, is the eighth member of San Diego's lone Super Bowl team who has died, all before the age of 45. Lew Bush, Shawn Lee, David Griggs, Rodney Culver, Doug Miller, Curtis Whitley and Chris Mims are the others. Causes of death ranged from heart attacks to a plane crash to a lightning strike.
Seau's death also is among a few recent, unexpected deaths of NFL veterans.
Duerson's family has filed a wrongful death suit against the NFL, claiming the league didn't do enough to prevent or treat concussions that severely damaged Duerson's brain before he killed himself in February 2011.
Former Atlanta Falcons safety Ray Easterling, who had joined in a concussion-related lawsuit against the league — one of dozens filed in the last year — died last month at age 62. His wife has said he suffered from depression and dementia after taking years of hits.
Seau is not known to have been a plaintiff in the concussion litigation.
However, his ex-wife told The Associated Press that Seau sustained concussions during his career.
"Of course he had. He always bounced back and kept on playing," she said. "He's a warrior. That didn't stop him. I don't know what football player hasn't. It's not ballet. It's part of the game."
Gina Seau said she didn't know if the effects of concussions contributed to Seau's death.
When Humphries joined the Chargers in a 1992 trade, he said it was obvious Seau was "the person who had the most energy, the most excited, the guy who tried to rally everybody." Humphries said Seau "brought out a lot of youngness" in older players.
He also helped younger players.
"So sad to hear about Jr Seau," tweeted New Orleans Saints quarterback Drew Brees, who was with San Diego from 2001-05. "Junebug. Buddy. The greatest teammate a young guy could ask for. This is a sad day. He will be missed greatly."
Seau called many of those around him "Buddy." He often referred to teammates as "my players."
Seau was voted to a Chargers-record 12 straight Pro Bowls and was an All-Pro six times.
"We all lost a friend today," Chargers President Dean Spanos said in a statement. "This is just such a tragic loss. One of the worst things I could ever imagine."
Seau's greatest game may have been in the 17-13 victory at Pittsburgh in the AFC championship game in January 1995 that sent the Chargers to the Super Bowl. Playing through the pain of a pinched nerve in his neck, he spread out his 16 tackles from the first play to the second-to-last. San Diego was routed 49-26 in the Super Bowl by San Francisco.
Seau left the Chargers after the 2002 season when the team unceremoniously told him he was free to pursue a trade. He held a farewell news conference at the restaurant he owned in Mission Valley, and later was traded to Miami.
Seau retired a few times, the first in August 2006, when he said, "I'm not retiring. I am graduating."
Four days later, he signed with the New England Patriots. He was with the Patriots when they lost to the New York Giants in the Super Bowl following the 2007 season, which ended New England's quest for a perfect season.
Last fall, finally retired for good, Seau was inducted into the San Diego Chargers Hall of Fame.
His last season was 2009.
Patriots owner Robert Kraft recalled the tight hugs he got from Seau in the locker room following games.
"He may have been one of the most charismatic Patriots players in franchise history," Kraft said. "Today, the fans of the teams for which Junior played — San Diego, Miami and New England — lost more than a legendary football player. We lost our 'Buddy.'"
More than 100 people gathered outside of Seau's home, only hours after he was found dead. Families showed up with flowers and fans wearing Chargers jerseys waited to get news.