The Washington Post's Michael Birnbaum reports from Nice, France a day after at least 84 people were killed when Mohamed Lahouaiej Bouhlel, 31, drove a truck into a crowd celebrating Bastille Day. (Michael Birnbaum,Jason Aldag/The Washington Post)

Deep disagreements and anger about France’s security measures mounted a day after a Tunisian-born Frenchman rammed a 19-ton truck into crowds at a Bastille Day fireworks celebration, killing scores of people and injuring at least 200 more. The Islamic State has claimed responsibility for the attacks.

Frustrated crowds booed French President François Hollande and Prime Minister Manuel Valls on Friday when they visited the bloodied seaside walkway on the French Riviera, where a massive cleanup operation was under­way as makeshift memorials to the victims rose up.

“We now realize that there was no protection for us,” said Karim Lourahri, 22, a butcher who witnessed the truck mow down several people.

As France grapples with the trauma of the third major assault on its soil in 18 months, the question on the minds of many of its citizens is: Will we ever be safe from terrorists?

Many French on Friday questioned how the attacker could have swept past police checkpoints at a prominent event that clearly demanded high security. On another level, there was soul-searching once again about France’s overall security strategy. Some critics called for more robust measures, such as state-of-the-art X-ray machines and stricter anti-terrorism laws, while others said that better intelligence was the only way to keep the country safe.

Georgetown University faculty member Rollie Flynn spent 30 years working with the CIA and identifies the traits of a lone wolf terrorist attack and why they are difficult to police. (Erin Patrick O'Connor/The Washington Post)

The concerns over security threaten to derail the political future of Hollande, whose popularity is at an all-time low. The burgeoning ranks of his political opponents already are saying that any president who has allowed three of the nation’s deadliest terrorist attacks is failing on security.

“We cannot make war on terrorism with laws of peace,” said Rudy Salles, the deputy mayor of Nice, who criticized Hollande’s plans to end an official state of emergency at the end of July, which were announced just hours before the attacks and swiftly abandoned afterward.

Salles said he was shocked by Thursday’s apparently lax security.

“We just organized security for the Euro football championship,” he said, referring to a soccer tournament that ended Sunday, during which matches were held across France amid a state of high alert. “So I was sure that we wouldn’t have any problems here because it was a very focused event.”

There were barriers to prevent vehicles from entering the avenue, but they were flimsy, and police officers were not present at every barrier. Some witnesses said their bags were not checked for bombs. Some of the police assigned to the site were grouped in large clusters, particularly near the fireworks site, while others were distributed much more ­thinly.

Some witnesses suggested that the police were lulled into a sense of security after the soccer tournament took place without incident. “They thought everything would be fine, but this is the biggest event in French history,” said Serhan Korkmaz, 23, a driver. “They should have taken better precautions.”

Police were caught off guard by the speeding truck driven by Mohamed Lahouaiej Bouhlel, witnesses said. Officers shot him dead only after he had mowed down victims over a mile-and-a-quarter stretch of the walkway.

“What we don’t understand is why he could drive so far,” said Sonia Chemmka, 24, who was watching the fireworks display. “The police had many armed officers in the area where we were and where they probably expected something to happen — but my friends who were on the side of the promenade, where the man started to drive into the crowd, told me that they saw very few officers and nearly no police cars there.”

Other witnesses gave similar accounts.

“I would say that there was no overall lack of police officers — but they were not well organized,” said Laure Teresi, 24, a friend of Chemmka.

Chemmka and Teresi fled to the beach immediately after hearing gunshots about 10:42 p.m. They said they were lucky that there were not more attackers. “We did not see a single officer at the beach who could have protected us,” Chemmka said.

But other witnesses, and security analysts, said that no matter what security measures are in place, the government and police cannot prevent the act of a single man, as all accounts so far suggest that this was. Just look at what happened in Orlando, they said, where a single gunman killed dozens at a gay nightclub last month.

One major challenge in tracking potentially dangerous individuals, experts said, is that the path from petty crime to threatening radicalization can be sudden and hard to track.

“The problem with such individuals, they’re totally invisible until they get radicalized,” said Jean-Charles Brisard, a French counterterrorism expert. “We’re not following the full spectrum of the threat or the potential threat.”

He said that French intelligence capacities could be drastically expanded, should there be the will. One place to focus is prisons, he said, which were a major spur for the radicalization of the assailants in the Charlie Hebdo and the November attacks in Paris last year. Counter­terrorism agencies say they have major blind spots in their intelligence in France’s teeming penal system.

Bolstering hard security measures, such as more barriers along parade routes, would do little to avoid threats, Brisard said, since attackers can always shift their targets elsewhere.

“We have reached the maximum we could reach in terms of security measures,” he said, adding that security is now “more a qualitative problem than a quantitative problem.”

Rick Noack and Annabell Van den Bereghe contributed to this report.