A team of scientists who analyzed the brain tissue of renowned NFL linebacker Junior Seau after his suicide last year have concluded the football player suffered a debilitating brain disease likely caused by two decades worth of hits to the head, researchers and his family exclusively told ABC News and ESPN.

In May, Seau, 43 -- football's monster in the middle, a perennial all-star and defensive icon in the 1990s whose passionate hits made him a dominant figure in the NFL -- shot himself in the chest at his home in Oceanside, Calif., leaving behind four children and many unanswered questions.

Patients with CTE, which can only be diagnosed after death, display symptoms "such as impulsivity, forgetfulness, depression, [and] sometimes suicidal ideation," Lonser said.

Seau's family described to ABC News and ESPN a long descent into depression in the years prior to his death.

More than 30 NFL players have in recent years been diagnosed with CTE, a condition once known as "punch drunk" because it affected boxers who had taken multiple blows to the head. Last year, 4,000 retired players joined a class-action lawsuit against the league over its alleged failure to protect players from brain injuries.

The night before his death, Seau sent a text message to his ex-wife and children in which he simply wrote, "I love you." They were the last words anyone would hear from him.