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A 10-part miniseries titled “American Sports Story: Aaron Hernandez” premieres September 17 on FX, chronicling the rise and fall of NFL player Aaron Hernandez, the New England Patriots tight end who was convicted of murder in 2015 and committed suicide in his prison cell two years later.

After his death, Hernandez was diagnosed with chronic traumatic encephalopathy, also known as CTE. According to the Mayo Clinic, the disease can cause impulsive and aggressive behavior, emotional instability and suicidal thoughts.

A researcher who examined Hernandez’s brain said at the time that he had the most severe case of the brain disorder she had ever seen in a person his age, raising questions about the cause of his violence. Hernandez was 27 when he died. Researchers have found no direct link between Hernandez’s illness and his behavior.

Here you can learn all about CTE and Hernandez’s brain.

What is CTE?

According to the Mayo Clinic, CTE is an incurable degenerative brain disease that causes nerve cells in the brain to die due to repeated head injuries. According to the Cleveland Clinic, the disease does not appear immediately and worsens over time. The disease can lead to behavioral changes, cognitive impairment and other brain diseases such as dementia.

CTE typically begins in the front part of the brain, where most head trauma occurs, and then spreads to other regions over time, Dr. Jesse Mez, co-director of clinical research at Boston University’s Chronic Traumatic Encephalopathy Center, tells TODAY.com.

The effects of CTE may not become apparent for years because the disease tends to progress slowly, Mez says. However, more frequent head injuries can accelerate the spread, as can age and a higher genetic susceptibility to CTE.

CTE has not been studied as thoroughly as other degenerative brain diseases, Dr. Daniel Daneshvar, director of brain injury rehabilitation at Massachusetts General Hospital, tells TODAY.com. That’s in part because CTE can’t be confirmed until after death, because the brain must be examined under a microscope.

Therefore, it is impossible to say how common the condition really is, Dr. Julie Stamm, a neuroscientist at the University of Wisconsin-Madison who studies sports-related brain trauma, including CTE, tells TODAY.com.

According to the Cleveland Clinic, people who develop CTE often have a history of professional sports. Most confirmed cases of CTE are in boxers and football players, but research shows that CTE also occurs in soccer players, hockey players, wrestlers, rugby players and others.

In addition to athletes, Daneshvar says war veterans who have been exposed to explosions and people who have suffered repeated physical abuse are also more likely to develop CTE.

Although many questions about CTE remain unanswered, there is one finding that links all documented cases: “a history of brain injury or repeated head impacts,” Dr. Willie Stewart, a neuropathologist at Queen Elizabeth University Hospital in Glasgow, Scotland, tells TODAY.com.

How does CTE affect a person’s behavior?

CTE can affect a person’s behavior in several ways.

“Memory loss and cognitive changes are the most predictable symptoms of CTE, but other common symptoms include impulsivity, aggression, verbal and physical violence, a short temper, loss of control and depression,” Dr. Ann McKee, a neuropathologist and director of the Center for Chronic Trauma Encephalopathy at Boston University, tells TODAY.com.

CTE “has the greatest impact on the frontal cortex of the brain, which plays an important role in impulse control and other behaviors,” Mez says.

However, it is still too early in research to definitively prove that CTE is the cause of the problematic behavior of some affected people, emphasize Mez and Nowinski.

What did Aaron Hernandez’s brain scans reveal about his CTE?

McKee, who studied Hernandez’s brain, says she has studied over 700 cases of CTE and found that Hernandez’s brain was “the worst case of CTE I’ve ever seen in a person who died at that age (27).”

Hernandez’s brain was donated by his family to Boston University’s Center for Chronic Trauma Encephalopathy, where it was extensively studied. McKee and her colleagues determined that he had stage 3 CTE, with stage 4 being the most severe and usually discovered in much older people.

Research also shows that the longer someone plays tackle football, the greater the risk of CTE. Hernandez played football as a teenager, the New York Times reported.

However, it’s still impossible to say whether the severity of his CTE caused his violent behavior. “We can’t make a connection between pathology in a person’s brain and specific actions a person takes,” Daneshvar says.

McKee adds, “It is impossible to know exactly what effect the CTE had on him, as many factors likely contributed to his behavior and actions. … The CTE likely contributed to his depression and erratic, impulsive and violent behavior.”

How did Aaron Hernandez die?

Aaron Hernandez was found dead in his prison cell in April 2017. He had apparently committed suicide. He was serving a life sentence in Massachusetts after being convicted of the murder of Odin Lloyd.

According to the Mayo Clinic, research shows that people with CTE may be at increased risk of suicide.