A man and a woman have been arrested in Nice in connection with the Bastille Day lorry attack, French judicial sources have said.
More than 80 people died when a Tunisian man, Mohamed Lahouaiej-Bouhlel, ploughed the vehicle into celebrating crowds on Thursday evening.
Earlier, France called up 12,000 police reservists to help boost security in the wake of the killings.
The latest arrests bring the number of those held over the attack to seven.
The driver was shot dead by police when his vehicle’s path along the seafront Promenade des Anglais was eventually halted.
A source close to the investigation told French news agency AFP that the killer had visited the area on the previous two days to prepare for the attack.
Those detained by French police include Lahouaiej-Bouhlel’s estranged wife. The others have not been identified.
French President Francois Hollande has called the attack terrorism and officials have said investigators will seek to find out whether the Tunisian had links with extremist groups.
French Interior Minister Bernard Cazeneuve said Lahouaiej-Bouhlel seemed to have been “radicalised very quickly”.
So-called Islamic State said the attacker was acting in response to its calls to target civilians in countries that are part of the anti-IS coalition.