Dec. 2, 2012— -- The death of Kansas City Chiefs linebacker Jovan Belcher, the latest in a string of tragic NFL suicides, has left the player's teammates, coaches, family and friends wondering what could have led a man described as generous and caring to murder his girlfriend -- the mother of his 3-month-old daughter -- and then kill himself.
Kansas City police say Belcher, 25, shot and killed his girlfriend Saturday morning before going to the team stadium and and committing suicide by shooting himself in the head as he was talking to coaches.
"When the officers arrived, when they were pulling up, they actually observed a black male who had a gun to his head and he was talking to a couple of coaches out in the parking lot," Kansas City Police spokesman Darin Snapp told ABC News Radio. "As officers pulled up, and began to park, that's when they heard the gunshot and it appears the individual took his own life."
It's not yet clear what prompted Belcher's actions, but his suicide follows those of former NFL players Junior Seau, 43, and Dave Duerson, 50, both of whom died of self-inflicted gunshot wounds to the chest in the last two years.
The suicides of Seau, Duerson and a number of other NFL players have been blamed on concussions racked up from playing the violent sport, and a condition called chronic traumatic encephalopathy, or CTE, but that may not be the case for Belcher.
Chiefs chairman Clark Hunt said today that Belcher was "a player who had not had a long concussion history," even though he was a three-time all-America wrestler and a star on the football team at his West Babylon, N.Y., high school.
Seau's and Duerson's brains are both being studied at Boston University's Center for the Study of Traumatic Encephalopathy, where researchers have already learned that Duerson had CTE, which may have led to his suicide.
CTE is a progressive, degenerative disease found in people who have had brain trauma from repeated blows to the head, according to the Center. It includes brain tissue degeneration and a buildup of an abnormal protein called tao, resulting in symptoms including confusion, aggression, and depression. Ultimately, CTE results in dementia.
In 2006, former Pittsburgh Steelers player Terry Long killed himself by drinking antifreeze, and former Philadelphia Eagles player Andre Waters shot himself in the head. Both of them suffered from CTE.
Researchers at Boston University found evidence of CTE in 12 of the 13 professional football players' brains they received between 2008 and 2010, according to the university. CTE can also be found in hockey players, wrestlers, and boxers.
"Football is entertainment in which the audience is expected to delight in gladiatorial action that a growing portion of the audience knows may cause the players degenerative brain disease," ABC News' George Will wrote in a Washington Post column published Aug. 3 just before he appeared on "This Week with George Stephanopoulos."
Will cited Seau and Duerson in his column, both of whom committed suicide after 2010, adding that 62-year-old former NFL safety Ray Easterling committed suicide in April 2012. Esterling's autopsy revealed that he had dementia and depression brought on by CTE.
NFL players have become significantly larger over the last 30 years, which may cause more injuries, Will said on "This Week." Although there were only three NFL players who weighed more than 300 pounds in 1980, there were 352 in 2011, he said. Three of those players weighed more than 350 pounds.
"Repeated, small but repeated blows to the head, the brain floating in the pan in the skull, now we know causes early dementia and other problems," he said, adding that football will have to change from "down below" as parents refuse to let their children play a sport that could give them brain trauma.
Seau's death in May prompted NFL player Jacob Bell to quit the sport altogether, leaving behind his contract with the Cincinnati Bengals.
"We're getting so much money, so much glory, so much fame; we're boosting our egos so much by playing a sport that's violent and could later on risk our lives," Bell said in May.
CTE researchers from Boston University were unavailable, and do not comment on players' deaths until more research is obtainable.
The Associated Press contributed to this report.