The accident occurred during Tuesday's ninth stage but Loprais was unaware of it until after his arrival at the bivouac that night.
Organisers said he was devastated and supporting local authorities with their enquiry.
"As a result he could not continue the race. The rally organizers join him in offering their condolences to the family and friends of the spectator, Livio Sassinotti," they said in a statement.
Loprais said on his facebook page, which also showed video of the incident, that the 69-year-old man was taking photographs out of sight behind a dune.
"We had been already in bed but the Dakar stewards showed us video footage of an accident from the race that we hit a man by mistake," he said.
"He was injured, he felt nauseous after two to three hours and he had a heart attack while being transported to the hospital. One human life has ended, indirectly my fault because I was driving.
"I have to admit that me and my crew didn't know about it at all. We've got onboards and other videos that prove it but nothing changes that a human life ended... unfortunately he was standing under the dune and we didn't see him at all.
"I'm very sorry and this accident will be with me for the rest of my life."
