The French government has deployed 12,000 police reservists to help strengthen security across the country after Thursday’s attack in the resort city of Nice in which a heavily-loaded truck crushed at least 84 people to death.

The reservists called up by France include 9,000 military police officers and 3,000 regular police officers, BBC reported, adding that 120,000 police and military personnel have already been deployed across the country. France, on Saturday, began a three-day mourning period for the victims of the deadly attack claimed by the Islamic State group, also called ISIS.

Meanwhile, French Interior Minister Bernard Cazeneuve also appealed to “all French patriots... to join this operational reserve.”

Cazeneuve’s statement comes at a time when some media reports alleged that French authorities removed police vans blocking off the promenade in Nice just hours before the truck ploughed through a crowd. Although France had been on high alert and there were concerns that Nice was a likely target for terror attacks, only 60 officers were on duty, the Telegraph reported.

Claiming responsibility for the attack, ISIS issued a statement Saturday, saying its follower Mohamed Lahouaiej Bouhlel “carried out the operation in response to calls to target nationals of states that are part of the coalition fighting Islamic State.”

Early on Saturday, French police arrested three people in Nice in connection with the probe into Thursday’s attack. Some people questioned by police reportedly revealed that Bouhlel, who had no apparent interest in religion, had recently gone through a rapid radicalization.

While authorities have not yet produced evidence that the 31-year-old Tunisian driver, who was shot dead by police, had any links to ISIS, French Prime Minister Manuel Valls said Bouhlel had recently been radicalised, and that there was no doubt about the motives behind the attack.

“The investigation will establish the facts, but we know now that the killer was radicalised very quickly,” Reuters quoted Valls as saying in an interview with Sunday newspaper Le Journal du Dimanche. “The claim on Saturday morning by Islamic State and the fast radicalization of the killer confirms the Islamist nature of this attack.”

However, some experts believe that ISIS’ claim does not necessarily establish a direct link between the incident and Islamic extremism.

“Islamic State called for such (individual) attacks to be carried out back in 2014. They are also using the public perception that an attack like this seems to fit Islamic State,” Edwin Bakker, a professor at the Centre for Terrorism and Counterterrorism at the University of Leiden in the Netherlands, told Reuters. “Investigators still have not discovered a direct link between Islamic State and the attacker, so it is a cheap claim.”