Seau, 43, among the best linebackers in NFL history, was found by his housekeeper with a gunshot wound to his chest, the North County Times reported, citing an unnamed source.
A former USC star, Seau was selected fifth overall in the 1990 draft by San Diego. He played for the Chargers from 1990-2002, then spent three years with the Miami Dolphins before finishing his career with New England. He played for the Patriots from 2006-09.
Seau made the Pro Bowl 12 times and was a 10-time All-Pro.
“Everyone at the Chargers is in complete shock and disbelief right now. We ask everyone to stop what they’re doing and send their prayers to Junior and his family,” the Chargers said in a statement.
In October 2010, he drove off a beachside cliff in Carlsbad in his Cadillac SUV and was found in the car on the beach, about 100 feet below the roadside. Hours earlier, he had been arrested on suspicion of domestic violence involving his live-in girlfriend.
Seau later said he was not trying to kill himself but had simply dozed off at the wheel.
On the surface, judging by initial reports, similarities can be drawn between the deaths of Seau and former Chicago Bears safety Dave Duerson, who died of a self-inflicted gunshot to the chest last year. In a suicide note, Duerson had asked his family to donate his brain to the Boston University School of Medicine.
Researchers from that school later determined Duerson suffered from a neurodegenerative disease linked to concussions, and that played a role in triggering his depression.