Chicago Bears legend Dick Butkus died after suffering a stroke last month, according to the death certificate obtained by TMZ Sports.
We broke the story ... the Hall of Fame middle linebacker passed away on October 5 in his Malibu, CA home, and now we know what happened to the beloved football star.
The certificate, issued by Los Angeles Department of Health, shows "cerebrovascular accident" -- known more commonly as a stroke -- as the immediate cause of death.
There were secondary causes listed as well ... including atrial fibrillation (an irregular heartbeat) weeks prior to his death, and dealing with high cholesterol and atherosclerosis (thick/hardened arteries) for years.
The document also points out Butkus underwent coronary bypass surgery in 2001.
Fittingly, Butkus' occupation is listed as "linebacker" ... considering he was one of the best to ever play the position. His business, no surprise, is the National Football League, where he dominated for nine seasons.
As we previously reported ... paramedics responded to a call for a patient in cardiac arrest at Butkus' residence in Malibu at 12:51 PM. He was pronounced dead at the scene.
Butkus' death sent ripples through the football world ... with everyone from legends to former stars weighing in on the loss of one of the most feared players to step on the gridiron.