Mohamed Lahouaiej Bouhlel, a Tunisian-born immigrant, was behind the wheel of a truck that barreled into Bastille Day revelers and claimed 84 lives. (Thomas Johnson/The Washington Post)

The man who plowed a truck down a crowded Nice thoroughfare and killed 84 people was a “soldier of the Islamic State,” the militant group said Saturday, as French authorities said the attacker was inspired by terrorist organizations.

It remained unclear whether the Islamic State had directed the attack, was taking responsibility for an assault it inspired or was simply seeking publicity from an event in which it had no direct hand. But no matter the exact connection to organized groups, investigators appear to believe that Mohamed Lahouaiej Bouhlel, 31, was taking his cues from their message.

The link underscores the difficulty of preventing the spread of extremist ideology in a world where even people like Bouhlel — whose family and neighbors portray him as a troubled loner — can be spurred to attack without training, resources or connections.

“It seems that he radicalized his views very rapidly. These are the first elements that our investigation has come up with through interviews with his acquaintances,” Interior Minister Bernard Cazeneuve said Saturday, without offering details. Five people have been detained for questioning in the case.

“We are now facing individuals who are responding positively to the messages issued by the Islamic State without having had any special training and without having access to weapons that allow them to commit mass murder,” Cazeneuve said.

A large truck rammed into a crowd, killing at least 84 people, French officials said.

The Amaq news agency, which is linked to the Islamic State, cited an “insider source” in declaring that Bouhlel “was a soldier of the Islamic State.”

“He executed the operation in response to calls to target citizens of coalition nations that fight the Islamic State,” the news agency said.

Separately, the Islamic State’s al-Bayan radio station said Bouhlel used “a new tactic” to wreak havoc. “The crusader countries know that no matter how much they enforce their security measures and procedures, it will not stop the mujahideen from striking,” the station said.

But the oblique claim of responsibility left open the question of whether Bouhlel had acted alone or had any prior communication with the group, which has also claimed ties to the attacks that struck Paris twice last year and Brussels in March. French authorities have been scrambling to determine whether Bouhlel had a support network in Nice, where he appeared to have been living for at least six years. 

Investigators on Saturday detained three people, including one person thought to have spoken to Bouhlel by phone minutes before he started his deadly journey down Nice’s Promenade des Anglais during Bastille Day celebrations. Another man was detained late Friday, according to the office of Paris prosecutor François Molins, and authorities detained Bouhlel’s ex-wife on Friday for questioning.

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

One focus of the inquiry is Bouhlel’s cellphone, recovered in the cab of the white truck that the Tunisian-born resident of France used to kill 84 people and injure 202. The contacts and call records in the phone can be used to stitch together a portrait of those who may have spoken to Bouhlel in his final days and hours. He was killed by police during the attack.

Nice, meanwhile, was trying to return to normal on Saturday by partially reopening the seaside Promenade des Anglais to traffic. Beaches also reopened, creating a jarring contrast between the tourists frolicking in the gentle Mediterranean surf and the blood-stained pavement above. Mourners dropped flowers on the spots where people had fallen, a route that stretched for more than a mile.

In Paris, President François Hollande convened an emergency meeting of his top security advisers to discuss the investigation.

“The ideologist of Daesh, Abu Muhammad al-Adnani, has for several weeks been repeating that it was necessary to attack directly, even individually, French people and Americans wherever they are and by whatever means,” said Defense Minister Jean-Yves Le Drian, using an Arabic name for the Islamic State. “Clearly, certain individuals, such as the driver of that truck, individually responded to this call for committing murder.”

“Even if Daesh doesn’t do the organizing, Daesh inspires a terrorist spirit against which we are organizing,” he said.

The attack has put the deeply unpopular Hollande on the defensive. Right-wing opposition politicians have pressed hard over his handling of security and terrorism.

Christian Estrosi, the center-right president of the regional council of Provence-Alpes-Cote d’Azur, which includes Nice, wrote in an open letter Saturday that he had asked two weeks ago for additional security during the Bastille Day celebrations. National authorities had turned down the request, he said, because there was no “special alert.”

But Hollande and his allies have rejected any notion that they erred in their security assessments.

“We are at a moment where there are temptations to divide our country,” said Hollande spokesman Stéphane Le Foll.

Cazeneuve said authorities had foiled terror plots connected to the European soccer championships that concluded a week ago.

Late Saturday, he urged “all French patriots” to join the national reserve, a measure of the sharp personnel need as French leaders scramble to get more security forces onto streets.

As authorities worked to determine whether Bouhlel was connected to a wider extremist network, an emerging portrait of the killer on Saturday suggested that he was a troubled and isolated man, never fully at home in any of the places, communities and families he had known.

Bouhlel had “psychological problems that caused a nervous breakdown,” his father told French television in comments broadcast Saturday.

“He would become angry, shout, break everything around him,” Mohamed Mondher Lahouaiej Bouhlel said in the family home town of Msaken, about 75 miles south of Tunis. “We had to take him to the doctor.”

Bouhlel’s arrival in France in 2009 or 2010 — his father could not recall the exact year — did not appear to have given him any peace or stability. He built a rap sheet for petty theft and assault. Although he married and had children, he allegedly beat his wife until she threw him out of their apartment in a low-income complex on the north side of Nice, according to neighbors.

After the split with his wife, he found an apartment in Nice’s Abattoirs area, a working-class district named after the city’s former slaughterhouse. It was on an anonymous thoroughfare in a peeling building next to a string of empty storefronts and parking lots.

Neighbors there said they were afraid of Bouhlel. Jasmin Corman, 38, said he had “fixed eyes” that terrified her and her two children, 14 and 7.

“He was always alone,” she said.

Standing in the doorway of her apartment on the building’s ground floor — directly below Bouhlel’s first-floor apartment — she recounted on Saturday how he recently stood on the stairwell and silently stared at her as she was locking her door.

“It’s horrifying to realize you were living beneath a murderer,” she said.

Corman, a Muslim who observes Ramadan, said that throughout the Muslim month of fasting, Bouhlel smoked and drank, occasionally returning to the building smelling of alcohol. For Muslims, such behaviors are strictly taboo.

She also noticed him outside the building with a young blonde on more than one occasion. “It was not his daughter. There were caresses,” she said.

Rebab Bouhlel, his sister, said her brother rarely called home to Tunisia but had begun to do so more often recently. “Over the past month,” she told Reuters, “he was calling us every day and he sent us money.”

“He called several times a day,” she said.

According to Tunisian media, one of those calls was made the day of the attack. Bouhlel reportedly called his brother to tell him about his troubles, mostly his divorce. Soon, he said, he planned to return to Tunisia.

Rick Noack and Annabell Van den Burghe in Nice and Souad Mekhennet in Frankfurt, Germany, contributed to this report.