May 2, 2012 -- The circumstances coming to light about the death of former NFL linebacker Junior Seau may highlight what some doctors see as a growing link between head trauma, mental illness and suicide, a connection that has come to the forefront of sports safety research in the last decade.
Seau was found dead from a gunshot wound to the chest at his home in Oceanside, Calif. Wednesday morning.
If Seau did indeed commit suicide, his death would bear a resemblance to that of other athletes in hard-hitting sports, including Chicago Bears football player Dave Duerson. Duerson shot himself in the chest in February of last year.
Seau played in the NFL for 20 years for the San Diego Chargers, Miami Dolphins and New England Patriots. On Wednesday, Chargers Chaplain Shawn Mitchell told ABCNews.com that Seau died of a "self-inflicted gunshot wound to the chest this morning." Seau was 43 and leaves behind three children and an ex-wife, Gina Deboer.
The Chargers released a statement to ABC News' San Diego affiliate: "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 case may be similar to that of Duerson, who left a note requesting his brain be sent to the "NFL brain bank" for study.
Several former NFL players have committed suicide in recent years, and many experts believe the deaths could be related to repeated blows to the head. In addition to Duerson, ex-Pittsburgh Steelers offensive lineman Terry Long and Philadelphia Eagles defensive back Andre Waters took their own lives. Chronic traumatic encephalopathy, or CTE, a degenerative and progressive disease found in people who have experienced repeated hits to the head, has shown up in the brains of several former athletes who committed suicide, including Duerson.
CTE has similar brain features to that of Alzheimer's, Parkinson's and Lou Gehrig's disease.
Last May, Dr. Ann McKee, co-director of Boston University's Center for the Study of Traumatic Encephalopathy, a research center that studied Deurson's brain after his death, told reporters that Duerson "had classic pathology of CTE and no evidence of any other disease," ESPN reported at the time.
"Exactly how the brain damage causes mood disturbance is not clear," said Dr. John Whyte, director of the Moss Rehabilitation Research Institute in Philadelphia, who does not know Seau's medical history. "There could be biological changes going on, or changes in the neurotransmitters that affect mood, or it could be a psychological factor that this brain injury has disrupted work and family life so much that it has really changed your life."
Until more research has been done, Whyte said the public should not jump to conclude a definitive link between concussions that Seau may have experienced in his career and death. But repeated blows to the head may also damage parts of the brain that have to do with impulse control and the ability to weigh the long-term consequences of decisions.
"Some people may feel really bad one day, but they can say, 'OK, this thought is out of proportion with reality,'" said Whyte, "whereas, if you're acting on impulse to certain emotions, you may feel bad one day and that can lead you to take action."
A concussion is caused when the brain is shaken so hard that it hits the inside of the skull, resulting in brain trauma. Studies have contributed to the growing concern over head injuries, particularly concussions, in football and other contact sports.
For reasons that remain unclear to experts, having one concussion makes a person more prone to further concussions. According to a study published in Neurosurgery last year, American football players who sustained three or more concussions were significantly more likely to develop depression and had were five times more likely to develop Alzheimer's disease.
According to statistics from the National Center for Injury Prevention and Control, about 47 percent of high school football players sustain at least one concussion each season. And 35 percent of those who reportedly suffered from a concussion actually sustained two or more in the same season.
The Centers for Disease Control and Prevention estimates that nearly 2 million brain injuries are suffered by teenage players every year.
While there is more concern over players and suicide, Whyte said sports are likely becoming less dangerous because of all the research being devoted to safety guidelines, proper equipment and the aftermath of a career of brain injuries.
In 2009, the NFL instituted new rules that require clearance from independent neurologists to allow players who suffered concussions to return to the field. The league also imposed stricter guidelines to reduce the number of helmet-to-helmet hits.
"The focus on sports safety has become much more vigilant about brain injuries and more strict with return-to-play guidelines," said Whyte. "But we certainly need more research to confirm whether these athletic-related injuries are leading to suicides."
ABC News' Sheila Marikar and Katie Moisse contributed to this report.