Police also found three telephones, three knives “and in particular a bloodied knife lying on the table,” Francois Molins told a news conference.
The killer, 25-year-old Larossi Abballa, had pledged allegiance to the Islamic State group three weeks ago, Molins said.
During the assault, he attacked the 42-year-old police officer, stabbing him repeatedly outside his home on Monday, before going inside and slitting the throat of his 36-year-old partner, who also worked at a police station.
Earlier on Tuesday, a police source said two people had been detained in connection with the attack, and Molins confirmed that a third person had also been taken into custody as part of the investigation into Monday’s double murder.
Molins also said wiretap surveillance of Abballa had given no clue to the upcoming attack.
He had been under surveillance since January as part of an investigation into a Syrian jihadi network.
Abballa was sentenced in September 2013 to two-and-a-half years in prison over his role in a jihadist group with links to Pakistan, but freed because of time already served awaiting trial.