Why?

Five days before his death, Seau was out with Vaughn Parker, a former Chargers teammate. Parker's lasting image of that evening was how Seau beamed about the decision of his daughter, Sydney, to follow in his footsteps and attend USC, where Seau came to national prominence as an All-America linebacker before San Diego took him with the fifth pick in the 1990 draft.

Those close to Seau say he was not in financial trouble—his sports-themed restaurant in Mission Valley is popular with locals and a destination stop for tourists—and he showed no outward signs of depression or emotional pain. He cherished his four children, Sydney and sons Tyler, Jake and Hunter. The day before his death he talked with close friend Joe Stabb, a longtime workout partner. "In that excited voice that he has, he gave me this big 'Joe! Joe! What's going on?'" Stabb says. "He told me they were having a big party at the restaurant on Saturday, and I should come down at noon to watch the Kentucky Derby and then the [Floyd Mayweather--Miguel Cotto] fight. He was Junior. The next morning I heard what happened. I was sick."

According to family and close friends Seau didn't own a gun. (As of Monday, police had yet to determine who owned the weapon Seau used to kill himself.) He left no note, nor any voice mails. The day before he died he texted "I love you" to each of his kids and to his ex-wife, Gina, but friends say that was not unusual. He often told those close to him that he loved them and greeted women and men with a kiss on the cheek.

So why? "No one is going to know but Junior," says Leonard Mata, a native of Oceanside and a police-department lieutenant.

It is too early to draw a straight line, but the question must be asked whether Seau's two decades in the NFL contributed to his death. In the previous 15 months, retired safeties Dave Duerson and Ray Easterling committed suicide by shooting themselves. Both suffered from depression and dementia that they attributed to multiple concussions sustained as players.

Seau never appeared on an injury report with a head ailment, but it would be naive to think a 6'3", 250-pound linebacker whose primary purpose was, as he had said, "to inflict pain on my opponent and have him quit" never suffered from brain trauma. Gina Seau said after his death that he had suffered from concussions during his career. And Taylor Twellman, an ESPN analyst who was once Seau's neighbor in Oceanside (and who was forced to retire from pro soccer due to the effects of a head injury), told the network that Seau admitted to him he suffered from headaches associated with concussions.

Bette Hoffman, the executor of Seau's estate, said she planned to recommend to the family that scientists be allowed to study his brain for signs of trauma. If Seau is found to have been suffering from chronic traumatic encephalopathy (CTE), a brain condition associated with repeated head trauma, it could further shake a league embattled by the issue of brain injury.

That issue is headed to the courtroom. Nearly 1,800 retired players have joined in a total of 68 lawsuits against the NFL (and, in several cases, helmet manufacturer Riddell), claiming the league failed to properly treat their concussions or did not adequately inform them about the potential long-term consequences of repetitive head trauma. None of those plaintiffs, however, is of Seau's stature. He was a six-time All-Pro and the NFL's 1994 Man of the Year, and he is certain to be elected to the Hall of Fame when he is eligible in 2015. Many consider him the most dominant linebacker of his generation, a tackling machine who was quicker than some backs and stronger than some linemen. Seau's combination of instincts, acumen and timing made him a terror when he lined up over the A gap. Few others could read a play and anticipate the snap count as well as Seau.

But what also made him stand out, to teammates and opponents alike, was his willingness to give. It didn't matter if you were a first-round pick or an undrafted rookie; if you were hungry, ready to work and respected the game, Seau would feed you all that he had. When middle linebacker Orlando Ruff asked to learn from him after being signed by the Chargers as a rookie free agent in 1999, Seau told Ruff to show up at the weight room the next morning at 6:30. After running back LaDainian Tomlinson, the fifth pick of the 2001 draft, asked Seau, "How do I become the player I want to become?" Seau would frequently pull him aside on the field or in the locker room and explain how defenses would seek to stop him and where the creases would be.

"One day as a rookie [in 1994] I asked him why he practiced so hard," says Harrison. "He said, 'Rodney, I get paid to practice. I play the game for free.' He said anybody can go out in front of 70,000 people and get excited and play a game, but it takes a special person to practice at game speed. That changed my whole career, and my life. When I retired from the Patriots, coach [Bill] Belichick said I practiced harder than any player he had in 30 years of coaching. That was all because of Junior."