An autopsy Thursday confirmed that the former NFL linebacker died of a gunshot wound to the chest, the medical examiner's office said. The office said further details would be released in a final investigative report, which may take up to 90 days to complete.
Officials said they were awaiting a decision by the family on whether to turn over Seau's brain to unidentified outside researchers for study.
Seau died Wednesday in his home in suburban Oceanside.
Police Chief Frank McCoy said Seau's girlfriend reported finding him unconscious with a gunshot wound to the chest and lifesaving efforts were unsuccessful. A gun was found near him, McCoy said. Police said no suicide note was found and they didn't immediately know who the gun was registered to.
Seau's ex-wife, Gina, told The Associated Press on Wednesday that Seau sustained concussions during his 20-year career. She said she didn't know if the effects of concussions contributed to Seau's death. Seau starred with his hometown San Diego Chargers for 13 years before playing with the Miami Dolphins and New England Patriots.
Seau's death follows the suicide last year of former Chicago Bears player Dave Duerson, who also shot himself in the chest.
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 died in 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.
Boston University's Center for the Study of Traumatic Encephalopathy has analyzed the brains of dozens of former athletes, including Duerson's.
While saying it was saddened by Seau's death, centre officials would not say if they have reached out to the Seau family or would be interested in studying his brain.
"It is our policy to not discuss any completed, ongoing or potential research cases unless at the specific request of family members," according to a statement released by the centre. "Our primary goal is to learn more about the long-term effects of repetitive brain trauma by conducting meaningful scientific research. At this time our thoughts and prayers are with Mr. Seau's family, his many friends and former teammates."
AP Sports Writer Jimmy Golen in Boston contributed to this report.