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A delivery company employee with links to radical Islam beheaded his boss then tried to blow up an American-owned industrial gas plant in an apparent terror attack in France on Friday, officials said.

The onslaught began with a delivery van crashing through the gates of the Air Products plant in Saint-Quentin Fallavier, southeastern France, shortly after 10 a.m. local time (4 a.m. ET) and drove into gas cylinders, triggering an explosion.

President Francois Hollande said there was "no doubt" that the attacker intended to blow up the entire plant.

Paris prosecutor Francois Molins said Friday afternoon that four people had been arrested: the suspect, his wife, his sister and another accomplice.

The suspect was able to get into the factory because he'd often made deliveries to the plant, which produces volatile industrial gases, Molins said.

Security cameras captured the van driving to the gates, and the driver ringing the buzzer, Molins said. The van was let in, and a few minutes later, it sped toward a hangar where various containers of gas were stored. Then there was an explosion, the prosecutor said.

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Firemen arrived five minutes after the blast, Molins said, and came upon the 35-year-old attacker, who was trying to open containers of acetone. He refused to identify himself, but employees of the factory, who recognized him, told the firemen who he was.

Not long afterward, the decapitated body was found, and, near the van, a knife, Molins said. The victim's head was found on a railing of the factory's gates, framed by two flags, both inscribed with the "shahadah, an Islamic verse that says, “there is no god but God, and Mohammed is his messenger.”

A French police official told the AP that the alleged attacker — who French Interior Minister Bernard Cazeneuve named as Yassine Salhi — is a resident of the Lyon suburb of Saint-Priest. Three French officials said the victim ran a delivery company that employed Salhi; the victim's name was not released.

Cazeneuve told reporters at the scene that the suspect was known to authorities and previously had been investigated for radicalization but surveillance ended in 2008. Molins indicated that authorities picked up on him again last year.

France was already on high alert following the deadly January attacks on satirical magazine Charlie Hebdo and a kosher supermarket which left 17 people dead.

Hollande said Friday that security has been stepped up in the region, adding that the decapitated body was found with "inscriptions" on it and that the attack bore the hallmarks of terrorism.

A woman believed to be Salhi's wife was seen on local television being taken into police custody in Saint Priest hours after the attack, the AP reported.

Air Products, based in Allentown, Pennsylvania, said the situation at the factory had been "contained" and that all its employees had been evacuated from the site and accounted for.

CEO Seifi Ghasemi said he was "horrified and saddened by the attack," adding that all workers there had safely left the site.

The beheaded person was not a worker at the factory, Le Monde reported, citing prosecutors.

Hollande spoke of “the necessity to protect our values and never give in to fear and not to create any unnecessary division and suspicions [which] would be intolerable and unacceptable.”

Air Products supplies industrial gases to businesses and employs more than 20,000 people in 50 countries, according to its website.

— NBC News' Jake Cigainero contributed to this report