After the arrest of the main suspect, French police have interrogated the second suspect in an attack on a gas plant in the southeast of France. No group has yet claimed responsibility for the assault.
On Saturday, the French police interrogated a second suspected Islamist as they tried to piece together the attack on the site of the American company Air Products in Saint-Quentin-Fallavier, some 40 kilometres (25 miles) from Lyon.
President Francois Hollande called an urgent meeting with Prime Minister Manuel Valls and key ministers on Saturday.
The second man is being investigated on terrorism-related charges but his link to the attack is not clear. French prosecutor Francois Molins said on Saturday that there was no indication that Salhi had an accomplice with him during the assault.
Main suspect known to the authorities
Yassin Sahli, the main suspect, is being held by the French police for causing an explosion by driving a delivery vehicle into a warehouse containing bottles of dangerous chemicals at the Air Products factory.
Emergency services that arrived on the scene found a decapitated body and the severed head of a 54-year-old man who was Salhi's boss at the delivery company.
Firefighters overpowered Salhi before discovering the decapitated body.
The Interior Minister Bernard Cazeneuve said that Salhi had been known to security services for a number of years but did not have a criminal record. He had been investigated for links to radical Salafists but had never been identified as planning an attack. Cazeneuve also said that Salhi's radicalization has led the police to open a case on him in 2006, but it was closed in 2008.
'a wolf in sheep's clothing'
A co-worker described the man as "a wolf in sheep's clothing" to the French Radio RTL, adding that Salhi had spoken to him about Islamic State but had not tried to recruit him.
Salhi is believed to have been the driver of the car. According to the French local newspaper Le Dauphine Libere, he was arrested a t his home in Saint-Quentin-Fallavier.
So far, no jihadi group has claimed the French attack, which happened on the same day as a massacre at the Tunisian beach resort of Sousse and a suicide bombing in Kuwait.
Holland ordered security tightened to the highest possible level in the region ahead of the emergency meeting of key ministers.
The attack in Saint-Quentin-Fallavier is the second jihadist assault to hit France in six months.
The French military are involved in several campaigns in largely Muslim countries in Africa and also against Islamic State in northern Iraq, making the country a target for Islamists.
France is home to Western Europe's largest Muslim population.
Beheadings have in recent times become a trademark of the "Islamic State group" Islamist group, based principally in Iraq and Syria but also operating in North Africa.
ra/rc (AFP, dpa)