The pioneering researcher who discovered that Aaron Hernandez was afflicted with advanced chronic traumatic encephalopathy said Thursday that she cannot solve one of the great mysteries in American sports: why the Patriots phenom who seemed to have it all, including a $40 million contract, would murder a friend over a perceived slight and hang himself five days after a jury cleared him of killing two strangers who may have angered him at a nightclub.
But Dr. Ann McKee, a neuropathologist who directs the Boston University CTE Center, said Hernandez’s brain disease — the most severe the BU team has seen in an athlete so young — is known to manifest in harmful ways.
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“We can’t take the pathology and explain the behavior, but we can say collectively that individuals with CTE of this severity have difficulty with impulse control, decision-making, aggression, often emotional volatility, and rage behavior,” McKee said.
McKee said the BU team has autopsied 468 brains, and no one younger than 46 who was diagnosed with CTE had a brain as damaged as Hernandez, who was 27 at the time of his death in April.
McKee displayed a series of images and pointed to severe damage to Hernandez’s brain that she said was caused by repetitive head trauma. The damage included substantial atrophy, or loss of brain tissue.
“In every place we looked, it was classic CTE,” she said.
She said she received Hernandez’s brain in very good condition, which presented a rare opportunity to study the disease in a person so young. McKee said Hernandez was born with a genetic marker that is associated with neurodegenerative diseases such as Alzheimer’s disease and possibly CTE.
“He may have had some susceptibility to developing a more aggressive type [of CTE],” McKee said.
McKee’s diagnosis was made public last month by Jose Baez, a lawyer for Hernandez’s estate, when he announced plans to sue the NFL for allegedly failing to protect the former player from the potential consequences of head injuries.
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BU issued a statement at the time confirming that Hernandez had Stage 3 (of 4) CTE and released images of a brain exam that showed classic telltale signs of the disease.
The school also confirmed that Hernandez’s CTE was the most severe that McKee’s team has found in someone so young.
But McKee had not spoken publicly or fielded questions about her findings until Thursday as part of her presentation during an academic conference on CTE being held at BU.
More than 110 former National Football League players have been diagnosed with CTE, an incurable, degenerative neurological disease that scientists have linked to repeated head blows, and Hernandez was the youngest, at 27. He took his life in his cell at the state prison in Shirley, where he was serving life for the execution-style slaying of Odin Lloyd in 2013.
The conviction has since been voided under a state law that reverts a criminal case to its original status if a person dies before he has exhausted all his legal appeals.
Five days before his suicide, Hernandez was acquitted of charges that he shot to death Daniel de Abreu and Safiro Furtado on a South End street in 2012. But he had a record of violence that spanned many years before his death.
While no one can prove a causal link between Hernandez’s brain damage and his actions, there is little dispute that he displayed CTE symptoms associated with behavioral problems, such as aggressiveness, explosiveness, impulsivity, and suicide.
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Hernandez began playing football before he was 6 years old, competing at home with his older brother Jonathan, who would go on to captain the team at the University of Connecticut. In all, Hernandez played the sport for more than 17 years and absorbed thousands of head blows as his career extended from the youth football fields and high school of his hometown of Bristol, Conn., to the University of Florida, and the NFL.
A Globe review of Patriots injury reports in September found that Hernandez suffered only one documented concussion during his three years in the NFL. There was no record of him experiencing a concussion while he played for Florida, although he missed only one of 41 games because he reportedly failed a drug test for marijuana.
Researchers have found, however, that CTE is linked not only to concussions but the large number of subconcussive impacts a football player experiences. There is no evidence that drug use causes or contributes to CTE.
The Patriots knew Hernandez had behavioral problems when they drafted him in the fourth round of the 2010 draft. They understood why, as a first-round talent, he was available to them with the 113th overall pick: one pre-draft NFL analysis stated of Hernandez, “Self-esteem is quite low; not well-adjusted emotionally, not happy, moods unpredictable, not stable.”
A separate analysis described him as “living on the edge of acceptable behavior,” and the Wall Street Journal reported that Hernandez received the lowest possible score, 1 out of 10, for “social maturity.”
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The Globe reported in 2010 that he acknowledged to several teams at the NFL Scouting Combine that year that he began abusing drugs after his father died. His mother, Terri, had told USA Today a year earlier that Hernandez’s began behaving badly after his father died unexpectedly at age 49 of complications after a routine hernia operation.
“He would rebel,” Terri Hernandez said. “It was very, very hard, and he was very, very angry.”
As a 17-year-old freshman in Florida, Hernandez punched a bouncer at a bar over a disputed bill, allegedly breaking the man’s eardrum. Before he left Florida three years later, he purportedly photographed himself wearing red, the color of the Bloods street gang, while hoisting a semi-automatic handgun.
In 2012, Hernandez claimed he would turn his life around after he signed his five-year, $40 million contract extension with the Patriots and his daughter, Avielle, was born. He was engaged to his high school friend, Shayanna Jenkins, who lived with him in North Attleborough.
“I can’t be young and reckless Aaron anymore,” he told reporters.
By then, however, he had been at the scene of the double murder in the South End. And, in the months after he vowed to curb his recklessness, Hernandez’s behavior became increasingly troubling. In February 2013, four months before Lloyd’s murder, Hernandez allegedly shot another friend, Alexander Bradley, in the face after a night of partying in Florida, according to a lawsuit by Bradley that Hernandez settled in 2016.
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Police later responded twice in early 2013 to domestic incidents involving Hernandez and his fiancée. In one incident, Jenkins reported that Hernandez had cut his wrist punching a window and was “losing a lot of blood.” No charges were filed. Both events occurred in Hermosa Beach, Calif., where Hernandez had traveled to work out in the offseason with Tom Brady.
To Patriots fans, Hernandez and Rob Gronkowski, his fellow Patriots tight end, seemed bound for glory. Viewed as the best tight end combination in the game, they would have won Super Bowl rings in 2011 had the Patriots not squandered a 17-15 lead in a 21-17 loss to the New York Giants.
And Hernandez might have celebrated with Gronkowski after the Pats won Super Bowl XLIX in 2014 had he not gone deeper on his destructive path. In June 2013, three months after the Hermosa Beach incidents and less than a month after he accepted the Pop Warner Inspiration to Youth Award, Hernandez was charged with killing Lloyd, a semipro football player from Dorchester who was dating Shayanna’s sister, Shaneah.
Now, Hernandez, too, is dead — one of three former Patriots with CTE who have killed themselves. The others were Junior Seau and Dennis Wirgowski.
In total, six deceased Patriots have been diagnosed with CTE, including Dr. Bill Lenkaitis, Mosi Tatupu, and Kevin Turner. Seau’s estate, like Hernandez’s, is suing the NFL over his death. The CTE diagnoses are central to their claims.
Bob Hohler can be reached at robert.hohler@globe.com.