A decapitated head covered in Arabic writing was found at a U.S. gas company factory in France after two assailants rammed a car into the premises, exploding gas containers.
The attack, in which one person was killed and at least one was injured, bore the hallmarks of Islamist militants.
The French public prosecutor said the anti-terrorist section had been deployed to investigate what it described as a suspected organized attack.
Police sources said the decapitated head was found at the site, along with a flag bearing Islamist inscriptions.
Local newspaper Le Dauphine reported that the head was covered in Arabic writing. One suspect had been arrested and was already known to French intelligence sources, one of the sources said.
If confirmed as an attack, it would be the second major such incident in France this year, coming after Islamist gunmen killed 17 people in attacks on the offices of the Charlie Hebdo satirical weekly and a Jewish food store in January.
"Two individuals deliberately rammed a car into the gas containers to trigger an explosion," the police source said.
The site belonged to Air Products, a U.S.-based industrial gases technology company, according to a spokeswoman for Air Liquide, a French company in the same sector. It was immediately blocked off by police and emergency services.
French Interior Minister Bernard Cazeneuve was heading to the site in the town of Saint-Quentin-Fallavier, around 30 km (20 miles) southeast of the city of Lyon.
French media said the government had ordered security to be stepped up around sensitive sites in the surrounding Rhone-Alpes region.