Rob Lytle, a rookie hero on the Broncos’ first Super Bowl team in 1977, has died of an apparent heart attack.

He was 56.

“Unbelievable. Too young, way too young,” said Billy Thompson, who was Lytle’s teammate from 1977-81 and still works with the Broncos as their director of community outreach. “A great guy. I still can’t believe it. He was an all-around good athlete, a good friend. To hear that was just tragic news.”

According to a statement from Memorial Hospital in Fremont, Ohio, Lytle was brought by ambulance to the hospital Saturday night with heart-attack symptoms. It was while at the hospital that Lytle went into massive cardiac arrest and died.

A running back, Lytle was one of the best college football players in the country while at the University of Michigan, where he set school records with 3,317 rushing yards and 26 touchdowns. He finished third to Pittsburgh’s Tony Dorsett and USC’s Ricky Bell in the Heisman Trophy voting in 1976.

A second-round draft pick of the Broncos, Lytle in his rookie year had 408 yards rushing and a touchdown, plus another 198 yards receiving and another touchdown in the team’s magical 12-2 season. He also scored the Broncos’ first Super Bowl touchdown, although they lost to Dallas, 27-10.

In his seven NFL seasons, all with the Broncos, Lytle rushed for 1,451 yards with 12 touchdowns and had 562 receiving yards for two scores.