A French prosecutor says he suspects an act of cannibalism was carried out on a prisoner by a cellmate in the northern city of Rouen.
Cannibalism was "very likely", the prosecutor said after an autopsy showed that some organ parts were missing.
The suspected killer claimed he had eaten the victim's heart.
The prosecutor, quoted by the AFP news agency, said some muscle had been removed from the victim's rib cage and part of one lung was missing.
But the heart remained intact, the prosecutor said.
The suspected killer and another prisoner were sharing the cell with the victim when the attack happened on Wednesday, French TF1 television reported.
All three were awaiting trial accused of sexual crimes, the TF1 website said.
The suspected killer, aged 35, is believed to have beaten up the victim, 31, and then suffocated him with a plastic bag.