Nearby they found the draft of a novel titled "Cannibalistic Instincts," said the official, who spoke on customary condition of anonymity.
Jose Luis Calva told police he had boiled some of his girlfriend's flesh but that he hadn't eaten it, the spokesman said.
The official also confirmed other details released by the prosecutor's office Wednesday night: that body parts were spread throughout the apartment, and that Calva is being investigated in the deaths of three women whose mutilated bodies had been found in and around the capital.
The prosecutor's office said Calva tried to escape when officers entered his apartment, but was struck by a car and hasn't recovered enough for a formal statement to police.
Police had come to Calva's apartment Monday to investigate the disappearance of his girlfriend, Alejandra Galeana, a 30-year-old pharmacy clerk and single mother, according to the prosecutor's office.
The office said that parts of her dismembered body was discovered in a closet, the refrigerator and inside a cereal box.
Calva also is being investigated in the mutilation killings of an unidentified prostitute in April and an ex-girlfriend whose body was found in 2004, the prosecutor's office said.