George Gojkovich via Getty Images Kevin Turner died in March. He had initially been diagnosed with ALS in 2010.

Former NFL running back Kevin Turner suffered from the most advanced stage of chronic traumatic encephalopathy, or CTE, a degenerative disease linked to repeated brain trauma, when he died at age 46, researchers at Boston University announced Thursday.

Turner, who played eight seasons with the New England Patriots and Philadelphia Eagles, died in March of what was initially considered complications from amyotrophic lateral sclerosis, or ALS. But in a press conference Thursday, Dr. Ann McKee, the director of Boston University’s CTE Center, said Turner did not in fact have ALS. Rather, his advanced CTE was causing similar symptoms.

“The severity of Mr. Turner’s CTE was extraordinary and unprecedented for an athlete who died in his 40s,” McKee said in a news release.

Turner began playing football at the age of 5. Researchers who studied his brain believed the length of his career contributed to the development of the CTE. Research has shown that the length of a player’s career could be a key factor in the development of the disease later on.

“We believe the extreme severity of Kevin Turner’s disease is related to his 25-season career, and the fact he began playing tackle football at age 5, while his brain was still rapidly developing and more vulnerable,” said Dr. Robert Cantu, a Boston University neurosurgeon.

Turner was a plaintiff in the class-action lawsuit that more than 4,500 former players brought against the NFL, claiming that it mishandled concussion treatment and withheld information about the long-term effects of the injuries from players. The league reached a settlement of the lawsuit in 2015, but some players have since appealed.

Turner pledged to donate his brain to CTE research in 2010, when he was initially diagnosed as having ALS, or Lou Gehrig’s disease. He believed football had contributed to the development of the neurological disease.

“While learning the results has been devastating to Kevin’s family and those of us close to him, unfortunately, we cannot say we are surprised,” said Tamara Alan, the executive director of the Kevin Turner Foundation, which Turner founded in 2010 to raise awareness about ALS and brain trauma. “We will continue to honor Kevin’s wishes that we bring attention to this devastating disease.”

More than 95 percent of the donated brains of former NFL players showed signs of CTE in a 2015 study conducted by Boston University and the Department of Veterans Affairs.