French President Francois Hollande says authorities remain concerned about the American-owned gas plant attack last week by a man with links to Islamic extremists in Syria.

Hollande met with police in Lyon on Wednesday and the firefighter who apprehended the attacker.

Suspect Yassin Salhi was handed preliminary murder and terrorism charges, accused of decapitating his boss. His lawyer said he acted for personal reasons, but France's chief anti-terrorism prosecutor says Salhi had ties to radical Islam dating to 2003 and had sent photos of his employer's remains to a friend in Syria.

Hollande told the firefighter "a lot of concerns remain," saying the attack could have prompted a devastating explosion.

Hollande said authorities remain on high alert, although the terror alert level in the Lyon region itself will be lowered.