Canadian authorities had been assisting in the investigation after it emerged that Salhi had sent the image to a Canadian phone number. But officials had warned that the WhatsApp message could have been sent to a relay number and that the intended recipient could be anywhere in the world.
Salhi, a deliveryman and married father, drove his van into the American-owned Air Products factory near Lyon on Friday in what President Francois Hollande has described as a "terrorist" attack designed to blow up the building.
After a blast that damaged the warehouse and destroyed the roof and back of the vehicle, police discovered the severed head of Salhi’s boss, 54-year-old Herve Cornara, attached to the gates of the plant near two flags that prosecutors say had the Muslim profession of faith written on them.
He was overpowered by a firefighter on the scene and arrested.
The assault came six months after the Islamist attacks in and around Paris that killed 17 people starting with a shooting at satirical magazine Charlie Hebdo.
Salhi had been known to security services for a number of years and had been investigated for links to radical Salafists.
A memorial was held Saturday for Cornora, 54, in his hometown Fontaines-sur-Saône.