Smit, 75, who suffered from colon cancer, died Wednesday at a hospice in Colorado Springs, The Denver Post reported.
"He died peacefully with his family all around him," his daughter, Dawn Miller, said.
Family members said friends came, often traveling long distances, to see him in his last days. They included John Ramsey, who prayed with Smit years ago before a grand jury hearing and returned to Colorado last week to pray the detective's bedside.
Earl Aldrich, a former partner, remembered Smit as a deeply religious man who prayed with suspects before questioning them.
"I think they just like him," Aldrich said. "Lou had a knack for talking to these guys. They wanted to clear their conscience."
Smit had already retired when he agreed to help Boulder, Colo., police with the investigation into JonBenet Ramsey's death. The 6-year-old's body was found strangled in a basement room at her parent's home, in a case that is still unsolved. Her parents were suspects.
In 1995, Smit helped clear another man of killing his daughter and found evidence that led to the conviction of a serial killer.