Yassin Salhi was another extremist who fell through the cracks.

Like the three homegrown terrorists who staged multiple assaults in and around Paris in January, Salhi, a 35-year-old ­deliveryman who staged an audacious attack Friday at an American-owned chemical factory in southeast France, was well known to French intelligence. In 2006, he came under surveillance for his links to radical Islamist groups. But by 2008, authorities were following him less frequently, though they were still occasionally monitoring him between 2011 and 2014.

On Friday, Salhi, a father of three, drove a delivery truck onto the grounds of a chemical plant, left the severed head of his employer on a gate and then sought to spark a chain of explosions before being seized by firefighters. French authorities said two flags bearing Arabic inscriptions were found at the scene beside the severed head.

As a radical Islamist ideology emanating from the Middle East makes inroads in Europe, the attack suggested the difficult task confronting French and other European intelligence agencies. There are, experts say, simply too many possible suspects to track and too few agents to monitor them.

There were no immediate assertions of responsibility or possible motives given for Friday’s attack in this industrial zone about 13 miles southeast of Lyon. Authorities were probing a possible connection to other terrorist attacks Friday in Tunisia and Kuwait. Yet given the fact that the only victim was Salhi’s employer, it remained unclear whether there may also have been personal motives for the assault.

Nevertheless, it came as more than 1,200 mostly young French Muslims have left to fight for the Islamic State in Syria and Iraq, and as four French nationals over the past 13 months have staged brazen terrorist attacks on French and Belgian soil. The victims of those attacks included 17 people killed in January at the satirical newspaper Charlie Hebdo and at a Jewish grocery store in Paris.

“It’s a pure terrorist attack, especially inasmuch as a corpse has been found, decapitated with a message,” said French President François Hollande, who summoned security officials to an emergency meeting.

At a news conference, Paris prosecutor François Molins said Salhi entered the plant at 9:28­ a.m. “without attracting attention” because he had frequently delivered packages to the facility before. A security camera, he said, showed Salhi’s vehicle accelerating toward a covered shed, sparking an explosion. A part of the shed was destroyed in the explosion, the back of the vehicle was destroyed and the roof disintegrated.

Precisely what caused the explosion remained unclear, though significantly, it did not ignite a larger blaze.

By 10 a.m., firefighters had subdued Salhi, and he was taken into custody. Three other people were arrested later: Salhi’s wife and sister, both picked up at his home, and another person suspected of unspecified “criminal association.”

The victim’s headless corpse was found next to the vehicle. A knife was also found close by. In an area outside the range of the surveillance camera, the head was attached to the fence framed by two flags with Islamic professions of faith.

“The investigation will allow us to determine whether the decapitation occurred before or after the death of the victim, the motive of the killing and eventual complicity,” Molins said.

Although Salhi was subject to regular surveillance between 2006 and 2008, intelligence monitoring dropped off after that, Molins said. Authorities said Salhi had no criminal record. Still, between 2011 and 2014, the prosecutor said, Salhi drew the attention of the intelligence services from time to time because he had links with radical Islamist movements in Lyon.

Salhi, according to RTL radio, was also the subject of at least two French intelligence reports in 2013 and 2014. While residing in Besancon, in eastern France, he sought to create an Islamic institute with two friends who were “classified as extreme Muslims,” RTL reported. In 2014, the station said, Salhi’s neighbors reported to authorities that he was showing signs of radicalization. For instance, he reportedly hosted meetings at his house with men who would sometimes wear military clothing and, the station said, “talk about jihad.”

Reached before her arrest by France’s Europe 1 radio station, his wife appeared surprised.

“What is going on? I don’t understand,” she told a journalist. “My husband just went to work. He is a deliveryman. My sister-in-law called me this morning to say I had to watch the news on TV. I don’t understand. My husband just went to work. We have a normal life, we are a normal family.”

Jamel Khablech, 48, a neighbor living in the same building as Salhi in this quiet suburb of Lyon, said the attacker and his family had recently moved to the area from Besancon. Khablech said that Salhi’s three children would play with other kids who lived there but that Salhi kept to himself.

Salhi wore Western clothing, Khablech said, not religious garb. He wore no beard, though Salhi’s wife covered her hair with a veil. Khablech was at work when he heard the news of the attack, and he connected the dots after his wife informed him that journalists were calling neighbors to see if anyone knew Salhi.

Khablech said he immediately asked his wife to take their two children and leave.

“I was scared for my kids. When you realize your neighbor is a terrorist, who knows the kind of stuff he can hide in there,” he said.

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