PARIS—French authorities detained a fifth person Sunday in a terrorism investigation seeking to identify potential accomplices and motives after a police official was fatally stabbed at a police station outside Paris. French police killed the 37-year-old Tunisian attacker shortly after he stabbed the unarmed administrative employee on Friday at the entrance of her police station in the town of Rambouillet. In a news conference on Sunday, anti-terrorism prosecutor Jean-Francois Ricard said police are questioning a cousin of the suspect. The suspect’s father, a couple who had provided him with an address for mail and other administration, and another cousin were also being questioned, Ricard said. The victim, a National Police employee, had left the station to extend her time on a parking meter and was followed into the entry area and stabbed by the attacker. He was then shot to death by a police officer. The attacker, identified by authorities …