NICE, France >> There is mounting evidence that Mohamed Lahouaiej Bouhlel, the Tunisian-born truck driver responsible for the deadly carnage in Nice last week, had recently absorbed extremist ideas and had become radicalized, French authorities said Sunday.

The image of a religious extremist is at odds with the portrait that neighbors and family members initially painted of a man who ignored even the most basic rules of Islam, eating pork, drinking alcohol and shunning the mosque.

Many of those who knew him said in the days after Thursday’s Bastille Day attack that Bouhlel was a difficult person, describing him variously as aloof and hostile, even violent at times. In March, he received a suspended sentence for a road-rage incident — not enough to put him on the radar of France’s security services.

But officials said Sunday that the 31-year-old had apparently undergone a rapid conversion to radical Islam and carefully planned the attack that claimed the lives of at least 84 people, including 10 children, raising the question: how did a delivery driver go from petty crime to carrying out an act of mass slaughter in the space of a few months?

Hours after the Islamic State group claimed responsibility for the attack Saturday, Prime Minister Manuel Valls said authorities “now know that the killer radicalized very quickly.”

Neither IS nor the French government have provided tangible evidence of a link between the group and Bouhlel. But Valls told the newspaper the Journal du Dimanche in an interview Sunday that the extremist group “is encouraging individuals unknown to our services to stage attacks.”

“That is without a doubt the case in the Nice attack,” said Valls, warning that “terrorism will be part of our daily lives for a long time.”

While authorities have said little publicly about their investigation, a French security official told The Associated Press on Sunday that Bouhlel sold his car just before the attack, which ended only when he was killed by police.

Bouhlel rented the refrigerated truck on July 11, purchased a pistol and was seen on closed-circuit TV footage visiting the promenade in the following days, according to the security official, who wasn’t authorized to be publicly named speaking about an ongoing investigation.

On July 14, France’s national day, Bouhlel sent text messages to people who may have been accomplices, the official said. One of those who received text messages was among eight people taken into custody after the attack. The official wouldn’t comment on the content of the text messages or confirm reports that they included a request for more weapons.

At least two of the three people detained Sunday are suspected of helping Bouhlel obtain the pistol found in the truck, the official said.

Most of those taken in for questioning, including Bouhlel’s estranged wife, who has since been released, described him as violent and unstable. While they all said he had long been indifferent to religion, some described a recent and very rapid conversion to radical Islam, the official said, noting that the attack appeared clearly premeditated.

Experts say that Bouhlel would have moved in an environment where he would have been exposed to the extremist ideology preached by the Islamic State group and others.

According to Yasmina Touaibia, a political scientist at the University of Nice, the region is home to more than a dozen informal mosques known to spread radical and fundamentalist ideas. Along with Paris and Lyon, it has become one of the main recruiting grounds for jihadis who have left France to fight in Syria.

A lawyer for one of those detained by police said his client hadn’t recognized any signs of radicalism in Bouhlel.

Jean-Pascal Padovani said his client had known Bouhlel casually and consumed drugs with him in recent months. “(Bouhlel) wasn’t really a soldier of God who went to Syria and came back to France,” Padovani said. “He was a depressed person who used terrorism to justify this act.”

Brigitte Erbibou, a psychologist who has long worked in Nice, said Bouhlel’s reported lack of religious conviction may not have precluded a sudden embrace of extremism, noting that people who have resorted to violence in the past can apply that instinct in other situations.

“This quick flip to violence in the name of a political ideology becomes legitimate,” she said. “The instructions of IS are to act wherever one is, by whichever means one can, so this (attack) matches exactly the recommendations of IS.”

Sudden, extreme violence isn’t the reserve of religious extremists, of course. There are grim parallels to the case of Andreas Lubitz, the Germanwings pilot who crashed his plane into a mountain not far from Nice last year, killing all 150 people on board.

Investigators found Lubitz had researched how to lock his fellow pilot out of the cockpit before the crash and even briefly simulated the fatal descent on an earlier flight.

Both the Germanwings pilot and the driver in Nice received treatment for psychological problems in the past. Bouhlel’s father said after the attack that his son had been prone to violent episodes.

“Each time he had a crisis, we took him to the doctor, who gave him medication,” Mohamed Mondher Lahouaiej Bouhlel told BFM-TV.

U.S. Secretary of State John Kerry said Sunday that the challenge of finding and stopping people like Bouhlel was “worse than the needle in the haystack.”

If someone is an extremist of “one or two days vintage” it’s easy to cause mayhem, he said on CNN.