On a Monday this past summer, a man phoned a rental agency on the outskirts of Nice, on France’s shimmering Mediterranean coast, to inquire, in what the receptionist would later recall as hesitant French, about a large cargo truck. He would be moving to Montpellier in a week, on July 11, the man said. No trucks were available for that date. Perhaps his move could wait, the man replied, and he signed off without any apparent frustration. For a short time, then, calamity was deferred, though hardly anyone could have known this.

Some in Nice knew the man as one of the many playboy predators the city seems to beget—black hair slicked back off a shining brow, dress shoes tapering to varnished points, a dark shirt unbuttoned low to reveal the pectorals into which he had obsessively, unblushingly, invested himself. He was 31 but preferred older women, both for their erotic openness and, it seems clear, for their money. Those who knew him best knew him to be a cold and brutal man, detached, amused by little save rough sex and gore.

He lived inconspicuously enough, however, working as a deliveryman, driving a 13-ton truck. In late June, though, he had taken several weeks off, and now seemed to those he encountered to be restless and bored, or perhaps under the sway of some deepening madness, as several witnesses have testified. Had he been another man, he might have chosen to spend his vacation with his three young children, but they had never stirred in him any great tenderness, and to see them would have meant arranging things with his estranged wife. Rather, as usual, he pedaled his blue bicycle around the city, shot selfies, and phoned loose acquaintances, harassing them with calls and text messages. The man called his sister’s husband, whom he saw perhaps once a year. “I thought he wanted to hurt me, because I’m in serious conflict with his sister,” the brother-in-law told police. He agreed to meet the man but brought a screwdriver for protection. “He talked to me about his life,” the brother-in-law said, “about his work as a deliveryman, about women, about his big sex drive, about his need to seduce and sleep with women, about his father, whom he hates.” The screwdriver wasn’t necessary.

For much of his vacation, the man passed the time browsing the Internet. “Terrible deadly accident,” “horrible deadly accident,” “shocking video not for the faint of heart,” he typed. For months, he had been watching beheadings; he kept an image folder filled with corpses and viscera. The particulars—the identities of the dead, the motives of the killers—were not of any evident interest; any butchery would do. And yet much of the violence he watched was political violence. Perhaps this was inevitable. Never before has so much recorded sadism been so widely available. For this, the world has the Islamic State to thank. A man transfixed by blood would hardly have to subscribe to the jihadist group’s ideology to enjoy its work; it is undoubtedly the best on offer.

In the final two weeks of his life, however, and perhaps for the first time, the man appeared to develop an interest in Islam, the religion into which he had been born. He played recitations of the Koran in his car; he criticized a friend for listening to music; he began to grow a beard. Online, he researched the massacre at the Pulse nightclub in Orlando, a killing carried out in the name of the Islamic State.

Also in evidence on the man’s computer was his apparent fascination with the crowds drawn each summer to the Promenade des Anglais, on Nice’s tranquil coastline, where on July 14 the city’s Bastille Day fireworks can be watched unobstructed, reflected in the black mirror of the sea.

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On Tuesday, July 5, the man called the rental agency once again. By chance, the receptionist said, there had been a cancellation; a white Renault Premium, with a total hauling capacity of 21 tons, had become available. The man seemed to be thrilled, and in the afternoon visited the agency with his driver’s license and a deposit check for 1,600 euros. He introduced himself as Mr. Lahouaiej; the license identified him as Mohamed Salmène Lahouaiej Bouhlel, a Tunisian living in Nice. “He was very relaxed, very calm, and very attentive,” the receptionist said in a deposition, one of hundreds of documents included in the case file compiled by French counterterror investigators, which has not been released to the public. Lahouaiej Bouhlel returned six days later, on the morning of July 11, to pick up the truck. When he got back to Nice, he guided the white truck along the gentle curve of the Promenade des Anglais, a busy road and broad reddish walkway that runs along the Mediterranean, which glowed baby blue at that early hour of the day. He drove along the Promenade at least ten times more in the coming 72 hours.

Since 1880, France has held a national celebration on July 14, the day on which, in 1789, several hundred Frenchmen stormed the Bastille Saint-Antoine, a royal fortress and prison in eastern Paris, in what was effectively the first pitched battle of the French Revolution. (Only Anglophones call the holiday Bastille Day; the French simply call it the 14th of July.) The festivities often include military parades, and Lahouaiej Bouhlel photographed himself that day on the Promenade in front of military jeeps and a tank. At lunchtime he visited his aunt and uncle, who fed him melon and a summer salad of cooked peppers and tomatoes. Their relations were cordial, but his relatives found Lahouaiej Bouhlel to be bizarre and inscrutable. Lately, to his uncle’s confusion, he had been speaking favorably about the jihadists in Syria. “I didn’t understand his convictions,” the uncle said in a deposition that was also included in the confidential case file, “because he absolutely wasn’t religious. He didn’t even do Ramadan.” His aunt attempted to give him a watermelon to take with him. “Tomorrow,” he said. He told her he would be going to watch the fireworks that night.

At 9:34, Lahouaiej Bouhlel pedaled his bike to the white truck, which he had parked near his apartment. He put the bike inside and drove down toward the water, where the fireworks were to begin at ten. Tourists and locals alike, an estimated 30,000 in all, had crowded the beaches and the Promenade. The show ended at 10:20 or so; the streetlights lit up again, casting wan shadows on the sidewalk and the beach. It was a pleasant night, with a warm breeze and bouts of light rain, welcome after weeks of terrible heat. The crowds lingered.

At 10:32, Lahouaiej Bouhlel pulled onto the Promenade’s wide southern thoroughfare. He rode along with the traffic for 1,000 feet or so until, across from the children’s hospital that would soon receive the crushed and mutilated, he drove up onto the broad sidewalk, filled now with revelers and families. He had extinguished his headlights. Soon came the crack of exploding seaside benches, and the dull thud of bodies spinning off the front edges of the truck. Its driver grinned.

Mohamed Lahouaiej Bouhlel in 2015. matrixpictures.co.uk

Ali Charrihi spent the evening of July 14 on the Promenade with his parents, his wife and three young children, and his cousin Saïd. They watched the fireworks from the sidewalk above the Torrent de Magnan, an underground stream that flows down from the hills into the sea, and in which Ali, now 37, had sometimes played as a child. He and his mother, Fatima, had come together to Nice in 1984, from an isolated Berber village in the mountains of inland Morocco; his father, a factory worker and laborer, had moved to France 11 years earlier. Fatima, who arrived speaking only Berber and had never mastered much French, cleaned houses. She was a cherubic woman, with small, smiling eyes and round cheeks made to seem all the fuller by the hijab that ringed her face. She doted on her children, but also insisted on study and hard work, and could be severe. Her word was law, and that night she decided that the family would remain on the Promenade after the fireworks, to walk together.

Ali had double-parked his car, and his father was parked in a bus lane; Fatima sent them both away to re-park, and they hurried off with Ali’s two sons. That left four of them on the Promenade: Fatima, Saïd, and Ali’s wife, who was pushing her young daughter in a stroller. The girl asked to be picked up, and Saïd took her in his arms. Ali’s father drove past; Saïd pointed to the car, and the group watched it go by.

There was a loud crack, and Saïd turned to see the bench a few yards beyond them explode into splinters. “Truck!” he shouted. With the young girl in his arms, he leapt from the sidewalk down to the rocky beach below. The right edge of the truck passed inches from Ali’s wife’s face, and tore the empty stroller from her hands. Where Fatima had been, there was nothing.

The sidewalk was about ten yards wide, and revelers were fairly sparse on this stretch of the Promenade. Lahouaiej Bouhlel cut the wheel to aim at his victims, who, even when they were walking toward him, often failed to see him coming in the shadowy light. In his wake, the crowd began to scream and run, adults shoving children out of the way, children losing sight of their parents, panic driving the unharmed up and away from the Promenade and into the city.

For two kilometers, it drove no stampede, no rush to escape; the truck was rolling forward far more quickly than the wave of panic it set off.

The first police officer to see the white truck was a man named Christophe, an imposing military veteran with a clean-shaven head and a heavy gold ring. He called it in at 10:33, confusion and urgency in his voice: “We’ve got a truck that’s completely crazy, that’s driving on the street, that just ran people over!” He and two colleagues gave chase in their car, following the truck on the sidewalk, but in avoiding the bodies Lahouaiej Bouhlel left behind, they could not keep pace. They watched helplessly through the windshield.

Lahouaiej Bouhlel accelerated to perhaps 35 miles per hour. The crowds were denser in this section. At about one and a half kilometers, at 10:34, he struck Amie, a bubbly 12-year-old from Nice, out with another family for the festivities. At the children’s hospital, she told her parents that some kindly person had washed her face with towels from a beach club. She died within the hour. Amie’s father, cooking in the following months for himself, his wife, and his remaining daughter, found himself preparing four portions of every dish, though he used ingredients for only three.

The white truck accelerated to perhaps 55 miles per hour. It flew past a police barricade in the roadway; behind the metal barriers, the streets were open to pedestrians, and Lahouaiej Bouhlel turned left over the curb and off the sidewalk, into the crowd. Jean-Pierre Joussemet, a 78-year-old retiree, a small man with thin white hair and glasses, was pushing his 80-year-old wife in a wheelchair. They had retired to Nice, where Jean-Pierre was born, and lived in an apartment just off the Promenade des Anglais. She was in the advanced stages of multiple sclerosis, and he cared for her, lifting her in and out of bed, taking her out most days to walk the Promenade. They had gone out early enough to be sure that she would be in the first row of spectators, with nothing to obstruct her view of the fireworks. Their daughter found her sometime later, frightened and confused, at the Gelateria Pinocchio; her wheelchair was in the street, and Jean-Pierre was nowhere to be found.