(AP) France opened a terrorism investigation after an attack at a gas factory Friday left one person decapitated and several wounded, officials said. Flags with Arabic inscriptions were found nearby, and a severed head was posted on a gate at the factory's entrance.

Two French officials said the attack began midmorning when two men crashed a car into the entrance of a gas factory in Saint-Quentin-Fallavier in southeastern France, hitting gas canisters and touching off an explosion. The industrial site southeast of Lyon belongs to Air Products, an American chemical company based in Allentown, Pennsylvania.

One of the attackers was arrested, they said. It was unclear what happened to the other attacker. Both officials spoke condition of anonymity because they were not authorized to speak to the news media.

A security official said a severed head was found posted on the gate at the entrance to the factory, in what appeared to be an echo of Islamic State's practice of beheading prisoners and displaying the heads for all to see.

The official, who spoke on condition of anonymity because he was not authorized to release details to the media, said the body was found near the site of the explosion but that the victim was not decapitated by the blast.

Two flags, one white and one black, both with Arabic inscriptions, were found nearby, the security official said.

France's anti-terror prosecutor said an investigation was opened into the attack, which he said was carried out by "a terrorist group."

Aides reported that French President Francois Hollande watched TV news reports about the attack with German Chancellor Angela Merkel as both leaders attended a European Union summit in Brussels.

France went on high alert after attacks in January against the satirical newspaper Charlie Hebdo, a kosher grocery 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.