One person was found decapitated and two others injured Friday after an assailant drove a vehicle onto an industrial complex near the French city of Lyon and tried to blow up an American-owned gas factory.
"The intent was without doubt to cause an explosion. It was a terrorist attack," President Francois Hollande said at a news conference in Belgium.
French Prosecutor Francois Molins said four suspects were in custody, including the suspected attacker, identified as Yassin Salih, 35, his wife, his sister and another person close to him.
The attack targeted a chemical factory in the town of Saint-Quentin-Fallavier, about 20 miles southeast of Lyon in southeastern France. The plant is owned by the U.S. firm Air Products.
The slain victim's head was found yards away from the body, stuck on a perimeter fence, with Arabic writing on or near it. There were also flags or banners bearing Arabic writing found at the scene.
The newspaper Le Dauphine Libere reports that the decapitated victim was the manager of a transport company who happened to be at the site for a delivery.
Three unnamed French officials told The Associated Press that the victim was the employer of the suspected attacker.
Witnesses said a man drove into the forecourt of the gas supply company and spun his car around in the yard, reports CBS News' Elaine Cobbe. Then they heard an explosion and saw fire.
Molins said the vehicle entered the factory without attracting attention. The gate was opened normally because the driver was known to factory employees.
French Defense Minister Jean-Yves Le Drian declared the attack an "act of terrorism" and said the national Defense Council was to convene at the presidential Elysée Palace. Hollande cut short his visit to Brussels for a European Union summit and returned to Paris early Friday.
CBS News correspondent Elizabeth Palmer reports the factory was locked down along with other businesses in the area during the police investigation.
French Interior Minister Bernard Cazeneuve said the suspect, believed to be from the area, was in contact with a Salafist movement and had been flagged in 2006 for ties to Islamic extremists, but surveillance was dropped in 2008.
Cazeneuve said the man was detained Friday by an alert firefighter.
There were multiple reports that French law enforcement and intelligence agencies had indications in the days and weeks leading up to Friday's attack that such an incident could be in the offing.
Cazeneuve told reporters that multiple people have been detained. "People who could have participated in this abject crime are in custody," he told journalists.
A French prosecutor's office said anti-terror police in Paris were taking over the investigation, citing possible charges of murder, attempted murder, and criminal association with a view to committing terrorist acts.
The attack came shortly before a gunman opened fire on beachgoers in the Tunisian resort city of Sousse, killing 39, mostly European. In Kuwait, ISIS claimed responsibility for the suicide bombing of a Shiite mosque that killed 27 and wounded hundreds more. It's not clear if the attacks were related, but they come just days after ISIS called on followers "to make Ramadan a month of calamities for the nonbelievers."
Air Products said all its employees were accounted for after the attack. It has not confirmed whether staff members were among the two people reported injured and one killed. The company said in a statement that all employees have been evacuated from the site, which is secure.
It says, "Our crisis and emergency response teams have been activated and are working closely with all relevant authorities."
Air Products, which is based in Allentown, Pa., makes gases for a wide range of industrial uses, including for food production, medicine, and the oil and gas sector. It has more than 20,000 employees in 50 countries, mostly in the Americas, Europe and Asia.
The Hebdo ordeal culminated with two suspects cornered in a factory in the Paris suburbs, and both suspects, who were affiliated with al Qaeda's Yemen branch, being killed by police.
Separately, a man claiming allegiance to ISIS, Amedy Coulibaly, was killed in a police raid after taking hostages -- four of whom he killed -- at the Jewish market in eastern Paris.
ISIS and al Qaeda followers and supporters online have issued repeated calls for further attacks on French soil in the wake of the Charlie Hebdo attack.
A French Muslim convert warned of fresh attacks against the West in an ISIS recruitment video released June 8 by the group's branch in Raqqa, Syria.
"Keep insulting the honor of our prophet, and send your warplanes to bombard us, then you should expect attacks in your countries every week," the French ISIS militant, identified as Abu Salman, said in the video. He warned Europeans to draw lessons from the January attacks in Paris.