The Tunisian man who drove his lorry into crowds of people in Nice researched the route in the days before the attack, French media have reported.
The reports say Mohamed Lahouaiej-Bouhlel drove through the seafront promenade area of the French city on Tuesday and Wednesday in preparation.
More than 80 people died when he ploughed his vehicle into people celebrating Bastille Day on Thursday.
Six people are being held in connection with the killings.
The latest, an Albanian couple who have not been identified, were arrested on Sunday morning, French judicial sources said.
Lahouaiej-Bouhlel was shot dead by police when his vehicle’s path along the Promenade des Anglais was eventually halted.
Europe 1 radio said CCTV footage from the days beforehand showed him driving through the area in the lorry, closely observing the scene.
On the day itself, Lahouaiej-Bouhlel phoned his brother in Tunisia during the afternoon, sending a photograph of himself among the crowds, Reuters news agency reported.
“That last day he said he was in Nice with his European friends to celebrate the national holiday,” the brother, Jabeur, said, adding that in the photo “he seemed very happy and pleased, he was laughing a lot”.
France has called up 12,000 police reservists to boost security in the wake of the killings.
A BBC correspondent in Nice says the beaches and cafes are busy again and the promenade has been re-opened.
Many people have taken the opportunity to leave tributes or pay their respects to those, including 10 children, who were killed.
