Nice - France faced hard questions over security failings on Saturday after a Tunisian man rammed a truck into a crowd killing 84, as investigators tried to establish his motives.There has been no claim of responsibility for the attack in which Mohamed Lahouaiej-Bouhlel, 31, smashed a 19-ton truck into a mass of people celebrating Bastille Day in the Riviera city of Nice.French President Francois Hollande met with his defence and security chiefs and cabinet ministers as criticism from the opposition and media mounted over security failings after the third major attack in France in 18 months.READ: IS group claims Nice attacker as a 'soldier'"If we are at war, as the government tells us, then the currency of war is intelligence, learning from experience, analysing failures and victories," wrote Yann Marec in an editorial for the southern region's newspaper Midi Libre.He was one of several calling for action, and not merely "the same old solemn declarations" from the government, as Le Figaro daily said.Security questionsSome 30 000 people had thronged the palm tree-lined Promenade Des Anglais on Thursday night to watch a fireworks display with their friends and families but the night turned to horror as the rampaging truck left mangled bodies strewn in its wake.Hollande said the country would observe three days of mourning as he warned the death toll could rise further, with more than 50 people still fighting for their lives.Four more people linked to Lahouaiej-Bouhlel have been arrested. The driver's estranged wife is also being held by police.The massacre, which comes after two other major terror attacks in France within 18 months, has once again shaken the country to its core, raising questions over how to stop such an unsophisticated, yet deadly, assault.READ: Nice attacks - all the detailsWhile the attacker's motives were unknown, former prime minister and mayor of Bordeaux Alain Juppe said on Friday that the carnage could have been prevented if "all measures" had been taken.Government spokesperson Stephane Le Foll slammed Juppe's comments, saying there was as much security present for the fireworks display as there had been for the Euro 2016 football tournament in the city.He said there were more than 185 police, gendarmes and soldiers from operation Sentinelle, launched after January 2015 attacks in Paris, as well as municipal police and a vast network of surveillance cameras."Despite all of that, this man's decisions... created the drama and horror we experienced."‘Unknown’ to security servicesA French parliamentary inquiry last week criticised numerous failings by the intelligence services over attacks in January and in November 2015 which left a total of 147 people dead."We know of course that there are still flaws and shortcomings," said Hollande, who previously described the attack in Nice as being of an "undeniable terrorist nature"."Government intervention is imperative in that area in order to better coordinate our intelligence services," he added.Investigators were piecing together a profile of the driver, Lahouaiej-Bouhlel, a man with a record of petty crime and domestic violence, but no known connection to terrorist groups.Anti-terrorism prosecutor Francois Molins said the 31-year-old Tunisian was "completely unknown" to the intelligence services but that the assault was "exactly in line with" calls from jihadist groups to kill.For several years, extremist groups such as Islamic State and al-Qaeda have exhorted followers to strike "infidels" - singling out France on several occasions - using whatever means they have to hand.In September 2014, IS spokesperson Abu Mohammed al-Adnani, suggested supporters "run [infidels] over with your car".While some attacks on the west - such as the November assault on Paris and the March bombings in Brussels - were carried out by jihadists who have been to the centre of IS operations in Iraq and Syria, others have been led by so-called "lone-wolf" attackers.Inspired from afar by Islamist propaganda, such attackers are a massive headache for intelligence services.Lahouaiej-Bouhlel's father said he had suffered from depression and had "no links" to religion."From 2002 to 2004, he had problems that caused a nervous breakdown. He would become angry and he shouted... he would break anything he saw in front of him," Mohamed Mondher Lahouaiej-Bouhlel said in Tunisia.Neighbours described the attacker, who worked as a delivery man, as a loner who never responded to their greetings.He and his wife had three children, but she had demanded a divorce after a "violent argument", one neighbour said.In Nice, the seaside streets that would normally be bustling on a summer weekend were near-deserted, with teary residents making their way to the promenade to lay down flowers in memory of the dead.