LOS ANGELES (AFP) - Former NFL player Phillip Adams was suffering from the degenerative brain disease chronic traumatic encephalopathy (CTE) when he shot dead six people before killing himself in April, officials said on Tuesday (Dec 14).

A post-mortem examination of Adams' brain carried out by neuropathologists at Boston University found that the 32-year-old showed signs of "unusually severe" damage to his brain.

Ann McKee, the director of Boston University CTE Centre which conducted the examination, compared the findings to the case of Aaron Hernandez, the former New England Patriots star who was convicted of murder in 2015 before taking his own life in prison two years later.

"Phillip Adams had an extraordinary amount of CTE pathology in the frontal lobe, the area of the brain behind the forehead," McKee said.

CTE, which cannot be tested for in living individuals, is a degenerative brain disease caused by repetitive head trauma.

It has been linked to an array of behavioural symptoms including aggression, impulsivity, explosivity, depression, anxiety, paranoia, and suicidal tendencies, as well as progressive cognitive symptoms such as memory loss.

The discovery of CTE - which McKee said had been found in 315 former NFL players - led to the league's concussion crisis, culminating in a US$1 billion (S$1.3 billion) settlement to players in 2016.

York County Coroner Sabrina Gast told a news conference on Tuesday that Adams' family said the former cornerback had suffered "several concussions" during his playing career, which spanned six seasons between 2010 and 2015, and included spells with the San Francisco 49ers, Oakland Raiders and Atlanta Falcons.

Adams, who began playing gridiron in school, complained of "excruciating pain" from his injuries, as well as memory issues and difficulty sleeping, Gast said.

"Adams' 20-year career in football put him at a high risk for CTE," McKee told reporters on Tuesday.

Complex puzzle

Adams shot dead Robert Lesslie, 70; his wife Barbara Lesslie, 69; two of their grandchildren, aged nine and five, and two men working on an air conditioning system at the Lesslie home in Rock Hill, South Carolina on April 7.

Adams was found dead from a self-inflicted gunshot wound at a nearby home shortly after the slayings.

Gast said the CTE findings revealed Tuesday did not "in totality" provide an explanation to what had triggered Adams' rampage.

"But it does give us a small piece of a more complex puzzle that is still under investigation," Gast said.

A toxicology analysis of Adams found he had prescription amphetamines in his system, as well as the over-the-counter herbal extract kratom, which in lower doses works as a stimulant.

McKee meanwhile said it was not possible to say that CTE alone had triggered Adams' killing spree.

"It's difficult to say that it alone resulted in these behaviours because usually it's a complicated issue with many other factors," she said.

"But certainly we have seen this behaviour and it is in fact not what I would consider unusual in this disease."

A statement on behalf of the Adams family read at Tuesday's news conference welcomed the CTE findings.

"We are pleased to have a better understanding of the mental turmoil that Phillip was dealing with during the last moments of his life," the statement said.

"We cannot say that we are surprised by these results. However, it is shocking to hear how severe his condition was.

"We hope to bring awareness to this condition so players young and old can understand the risks... Phillip is not the first to battle with this disease and he will not be the last."