Sept. 23 (UPI) — Gale Sayers, the Hall of Fame running back who starred for the Chicago Bears in the 1960s, died on Monday. He was 77.

The Pro Football Hall of Fame announced Sayers’ death. His cause of death was not disclosed. Sayers was diagnosed with dementia in 2013.

“All those who love the game of football mourn the loss of one of the greatest to ever play this game with the passing of Chicago Bears legend Gale Sayers,” Hall of Fame President David Baker said.

“He was the very essence of a team player — quiet, unassuming and always ready to compliment a teammate for a key block. Gale was an extraordinary man who overcame a great deal of adversity during his NFL career and life.”

Sayers entered the NFL as the No. 4 overall pick in the 1965 NFL Draft. He played his entire seven-year career with the Bears. The five-time All-Pro and four-time Pro Bowl selection dealt with numerous injuries, which shortened his professional football tenure.

He was the 1965 Offensive Rookie of the Year and led the NFL in rushing yards in 1966 and 1969 before he was limited to just four games from 1970 through 1971 due to knee injuries. Sayers was the league’s all-time leader in kickoff return yards at the time of his retirement.

He was inducted into the Pro Football Hall of Fame in 1977 and remains the youngest person in history (34) to receive the honor.

Sayers’ friendship with Bears teammate Brian Piccolo — who died of cancer in 1970 — inspired Sayers book I am Third. The autobiography inspired the 1971 movie Brian’s Song.

The Wichita, Kan., native was inducted into the College Football Hall of Fame in 1977. His jersey No. 48 is retired at the University of Kansas and his jersey No. 40 is retired by the Bears.

Sayers also served as the athletic director at Southern Illinois University from 1976 to 1981 after he retired from the NFL.

“The entire Pro Football Hall of Fame family mourns the passing of Gale,” Baker said. “Our thoughts and prayers are with his wife, Ardie, and their entire family. We will forever keep his legacy alive to serve as inspiration for future generations.

“The Hall of Fame flag will fly at half-staff until he is laid to rest.”