Nice - France's interior minister boosted security measures across the country late on Saturday, calling upon thousands of reservists to help after facing severe criticism for alleged police and security failures in the deadly Bastille Day truck attack in Nice.The Islamic State group announced earlier that the Tunisian man who barreled through a holiday crowd with a truck in the southern French city was one of its "soldiers" - the first claim of responsibility for the attack that killed 84 people at a fireworks display.France on Saturday began three days of national mourning in homage to the victims - although that didn't stop politicians from sniping at each other over who bore responsibility for the failing to stop the attack.Interior Minister Bernard Cazeneuve announced he would call up 12 000 police reserves in addition to more than 120 000 police and soldiers already deployed around the country "because of the terrorist threat."The IS claim - circulated on social media by a news outlet affiliated with the group - didn't name Mohamed Lahouaiej Bouhlel, the 31-year-old Tunisian who authorities say was behind the wheel as a truck crashed into revelers Thursday night. But the statement quoting an IS security member said Bouhlel was following IS calls to target citizens of the countries fighting the extremists.The veracity of the group's claim couldn't immediately be determined, but French officials didn't dispute it.What is known publicly about Bouhlel so far suggests a troubled, angry, sometimes violent man with little interest in the group's ultra-puritanical brand of Islam. His own father, in Tunisia, said his son did not pray or fast for Ramadan. Neighbors described the father of three as a volatile man, prone to drinking and womanizing, who was in the process of getting a divorce.But in a statement to reporters, Cazeneuve hinted that Bouhlel may have had a last-minute adoption of a more extremist worldview."It seems he was radicalised very quickly," Cazeneuve said following a ministerial meeting at the Elysee Palace in Paris.It's also unclear whether or not Bouhlel, who was shot dead by police Thursday night, had been acting alone.The Paris prosecutor's office said Saturday that five people are in custody following the attack. The identities of four of them were not known, but neighbors told The Associated Press that Bouhlel's estranged wife had been taken away Friday by police.As French security chiefs met in Paris, Nice's seaside boulevard, the famous Promenade des Anglais, was slowly coming back to life. A makeshift memorial of bouquets, candles and messages had been set up near one end of the expansive avenue."It's satisfying to see life coming back," lawmaker Eric Ciotti told France's iTele broadcaster from the promenade. "It's a sign that life is coming back even if, naturally, nothing will erase those images of horror. ... Never since World War II has Nice ever seen such horror."But the suffering is far from over. Two days after the atrocity, some families were still hunting for missing loved ones, going from hospital to hospital to find people who had disappeared in the bloody chaos of the truck's rampage.Officials said 202 people had been wounded in the attack, with 25 of them on life support as of late on Friday.Regional council President Christian Estrosi - a member of France's opposition Republicans - described the country's current Socialist leadership as "incapable" in an open letter. He said he had asked that police be reinforced in Nice ahead of the fireworks display but was told there was no need.Cazeneuve disagreed, saying that high security had been assured in the region - including at the Cannes Film Festival and the Nice Carnival. Government spokesman Stephane Le Foll warned against attempts to divide the country, calling for "unity and cohesion."Still, the message was heard, prompting the security announcement later from Cazeneuve.France is heading into elections next year, and the deeply unpopular President Francois Hollande is facing multiple challengers, from within his own Socialist Party, from the right-wing Republicans and from the far-right National Front.