Former New England Patriots and Philadelphia Eagles fullback Kevin Turner spent the last several years of his life bringing awareness to amyotrophic lateral sclerosis, or Lou Gehrig’s disease, which doctors diagnosed him with in 2010.

But researchers at Boston University announced on Thursday that Turner, who died in March at just 46 years old, did not suffer from ALS but severe CTE, advanced further than researchers have previously seen in a player who died at such a relatively young age.

During a news conference, Dr. Ann McKee said an autopsy showed Turner had died of chronic traumatic encephalopathy; ALS and CTE can present similarly.

“This is not ALS; this is CTE,” McKee said. “The severity of Mr. Turner’s CTE was extraordinary and unprecedented for an athlete who died in his 40s.”

An Alabama native, Turner began playing football at a young age, and the sport took him to the University of Alabama and then to an eight-year career in the NFL spent with the Patriots and Eagles from 1992-1999. Doctors believe the decades Turner spent playing the game – and the thousands of collisions he endured – are the reason his CTE was so severe.

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

Turner isn’t the first athlete to be falsely diagnosed with ALS; McKee said former Boston College linebacker Ron Perryman, who died in 2011, was also told he had ALS only to have a postmortem autopsy show he had CTE.