A French prisoner has confessed to killing his cellmate and then eating part of the man's body, a prosecutor said Friday.

Nicolas Cocaigne — who was charged with cannibalism and murder — claimed he stabbed his victim with scissors and suffocated him with a plastic bag before removing his heart and eating it, according to the prosecutor's office in Rouen.

But an autopsy of the victim's body later revealed that two chest muscles and part of his left lung — not his heart — were missing, the office said.

"The absence of these two anatomical elements, which were not found at the crime scene, make the presumed killer's confession of cannibalism very likely true," Rouen prosecutor Joseph Schmit said.

Thierry Baudry's mutilated body was found Wednesday morning by a guard at the prison, in the northern city of Rouen.

Cocaigne, who was serving a four-year sentence for sexual assault, did not give a motive for the attack, Schmit said. Baudry had been in jail awaiting trial on sex-crime charges.

Cocaigne was charged Friday with premeditated murder and cannibalism, judicial officials said. A third cellmate who claimed he slept though the attack was charged with complicity in homicide, the sources said.

Cocaigne's lawyer, Fabien Picchiotino, said his client had been experiencing psychological problems, adding that he had requested last month that Cocaigne be placed in solitary confinement.

"He was handling his detention badly," Picchiotino said. "He should have been placed in a specialized [psychiatric] institution."