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A former US football player who police say shot dead six people in April has been found to have been suffering from a degenerative brain disease.
A post-mortem on Phillip Adams, who took his own life, revealed he had chronic traumatic encephalopathy.
Known as CTE, the disease is linked to repeated head trauma.
Officials say Phillips shot dead Dr Robert Lesslie, his wife and two of his grandchildren, along with two men working at Dr Lesslie's home.
The 32-year-old later killed himself after a stand-off with police in South Carolina.
As a professional US footballer he played in 78 NFL games for six different teams, including the San Francisco 49ers and New York Jets. He started playing the sport aged seven and retired from the NFL in 2016.
Police say his motive for the attack remains unclear but his family consented for his brain to be tested for CTE, which has been found in post-mortem tests on hundreds of NFL players.
A neuropathologist who studied Adams's brain revealed on Tuesday that he had stage two CTE - described as "unusually severe" - in the frontal lobes of his brain.
Dr Ann McKee, who serves as director of Boston University's CTE Center, said Adams' playing career had put him at "high risk" of the disease and compared his brain scan to that of another former NFL player, Aaron Hernandez, who took his own life while in prison for murder in 2017.
Researchers say the condition is linked to issues such as memory loss and aggression, as well as dementia at later stages.
A statement from Adams's family said they were not surprised by the results, but were shocked to learn about the severity of his injuries.
They have previously spoken about how his mental health had "degraded fast" before his death and blamed his playing career. They expressed hope in their statement that the findings would help "bring awareness" to CTE adding: "Phillip is not the first to battle with this disease and he will not be the last."
Police say Dr Robert Lesslie, his wife, Barbara and their young grandchildren, Adah and Noah, were killed by Adams at their home in Rock Hill, South Carolina earlier this year.
Two air-conditioning technicians working at the property, James Lewis and Robert Shook, also died in the gun attack.
York County Sheriff Kevin Tolson said Adams's connection to the family remains unclear. Officials found incoherent writings and more than 20 guns at his property and say Adams had medication in his system at the time of the attack.
"What we have here is a big puzzle," Sheriff Tolson was quoted by the York County Herald as saying. "We still don't know why he targeted the Lesslie family. The connection we know is Mr Adams lived nearby."