An attacker with suspected ties to French Islamic radicals rammed a car into a gas factory Friday in southeastern France, and a severed head was staked on a post at the entrance, officials said. France immediately opened a terrorism investigation.

Two people were injured, authorities said.

President Francois Hollande, speaking in Brussels, said the attack began shortly before 10 a.m. when a car crashed through the gate of a gas factory in Saint-Quentin-Fallavier, southeast of Lyon. The car then plowed into gas canisters, touching off an explosion, he said.

“No doubt about the intention — to cause an explosion,” Hollande said, calling the attack “of a terrorist nature.”

French Interior Minister Bernard Cazeneuve said a man who had been flagged in 2006 for suspected ties to extremists was seized by an alert firefighter, one of multiple people in custody after the attack. He said authorities were trying to identify the victim whose severed head was posted at the factory’s entrance.

“People who could have participated in this abject crime are in custody,” he said.

Cazeneuve said the suspect had been known to intelligence services, who had him under surveillance from 2006-08. The man is from the Lyon region, he said.

A severed head was found staked on a gate at the factory’s entrance, in what appeared to be an echo of the Islamic State group’s practice of beheading prisoners and displaying their heads for all to see. An official said two flags — one white and one black, both with Arabic inscriptions — were found nearby.

The official, who spoke on condition of anonymity because he was not authorized to release details to the media, said the victim was not decapitated by the blast.

France’s anti-terror prosecutor said an investigation was opened into the attack, and potential charges including plotting as part of “a terrorist group.”

Hollande spoke after watching TV news reports about the attack with German Chancellor Angela Merkel as both leaders attended a European Union summit in Brussels.

The industrial site belongs to Air Products, an American chemical company based in Allentown, Pennsylvania. The company would not confirm whether any employees were injured or killed.

“Our priority at this stage is to take care of our employees, who have been evacuated from the site and all accounted for,” the company said in a statement. “The site is secure. Our crisis and emergency response teams have been activated and are working closely with all relevant authorities.”

Cazeneuve said the investigation had just begun and cautioned against jumping to conclusions.

France went on high alert after attacks in January against the satirical newspaper Charlie Hebdo, a kosher grocery store and a policewoman that left 20 people dead in the Paris region, including three Islamic extremist attackers.

Since then, fears of copycat attacks have risen. One person was arrested after authorities said he was plotting to gun down churches in the Paris region. Cazeneuve said Friday that security has been heightened at religious sites around the country.

(AP)