OCEANSIDE, Calif. — Junior Seau, regarded as one of the N.F.L.’s best linebackers over a 20-year career with the San Diego Chargers, the Miami Dolphins and the New England Patriots, died of a gunshot wound to the chest Wednesday at his home in Oceanside, Calif. He was 43.


The Oceanside police said Seau’s death was being investigated as a suicide. He was found by his girlfriend in a bedroom of his beachfront house Wednesday morning, and a handgun was found near the body, the police said, adding that Seau did not appear to leave a note.


A native of Oceanside, Seau starred at the University of Southern California before being drafted fifth over all in the 1990 N.F.L. draft by the Chargers, who played 40 miles south of his hometown. A 12-time Pro Bowler, Seau played 13 seasons with the Chargers and was one of the team’s most popular players. In the 1994 season, he led the team to the Super Bowl, where they lost to the San Francisco 49ers, 49-26. The Pro Football Hall of Fame selected him for the 1990s All-Decade Team.


“Everyone at the Chargers is in complete shock and disbelief right now,” the Chargers said in a statement. “We ask everyone to stop what they’re doing and send their prayers to Junior and his family.”


Seau was traded to the Dolphins in 2003, and after three injury-plagued seasons he was released. He signed a one-day contract with the Chargers in August 2006 to announce his retirement. But four days later, he signed with the New England Patriots and was a member of the 2007 team that went undefeated in the regular season, losing to the Giants in the Super Bowl.


His last season in the N.F.L was 2009. He finished his career with 1,524 tackles, 56 ½ sacks and 18 interceptions.


He is survived by three teenage children: a daughter, Sydney, and sons Jake and Hunter.


“It is incredibly tragic and sad,” the N.F.L. said in a statement. “Our prayers are with Junior’s family.”


Seau is the second retired N.F.L player to commit suicide in the past few weeks. Ray Easterling, a safety for the Atlanta Falcons in the 1970s and a plaintiff in a high-profile lawsuit against the N.F.L. over its handling of concussion-related injuries, died last month of a self-inflicted gunshot wound.


The circumstances of Seau’s death raised comparisons to the former Chicago Bears star Dave Duerson. In February 2011, Duerson shot himself in the chest, saying in a note that he wanted his brain donated to the study of football head injuries.


Though remembered as a hard-hitting, inspirational linebacker, Seau did not have a documented history of concussions. He missed several games in his career with leg and chest injuries.


In October 2010, he sustained minor injuries when he drove his S.U.V. off a 30-foot bluff after being arrested on suspicion of domestic assault. The police said he fell asleep at the wheel.