The cause of death for the late Dick Butkus has been revealed.

TMZ Sports obtained the death certificate for Butkus, which showed that the former Chicago Bears legend died from a stroke.

An irregular heartbeat that Butkus had weeks prior to his death, as well as ongoing issues with high cholesterol and hardened arteries, were listed as secondary causes for Butkus’ death. Butkus had undergone coronary bypass surgery in 2001.

Butkus, who was 80, died on October 5 at his home in Malibu, Calif., where he was found unresponsive that day.

Butkus played for the Bears from 1965-73, and made eight Pro Bowls and eight All-Pro teams. Butkus also won two NFL Defensive Player of the Year Awards and was widely seen as the standard bearer of excellence at the middle linebacker position. He is the namesake of the Butkus Award, given every year to the top linebacker at the high school, college, and professional football levels.

Butkus was inducted into the Pro Football Hall of Fame in 1979 and had his No. 51 jersey retired by the Bears in 1994.