(CNN) At least 13 people who knew James Brown have said they want an autopsy or a criminal investigation of his death, according to a new CNN investigative series.

These skeptics include Dr. Marvin Crawford, who signed Brown's death certificate at an Atlanta hospital in 2006. In an exclusive interview with CNN, Crawford said he never believed that Brown died of natural causes.

"He changed too fast," Crawford said. "He was a patient I would never have predicted would have coded. ... But he died that night, and I did raise that question: What went wrong in that room?"

Officially, the Godfather of Soul died of a heart attack and fluid in the lungs at age 73 in the early hours of Christmas Day 2006. Twelve years later, his manager, his widow, his son, and several friends and associates all tell CNN there are serious questions about Brown's death that can only be answered by forensic investigators.

Read More