A 47-year-old man has been arrested in connection with the knife attack in a Nice church in which three people were killed, a French judicial source said on Friday.
The man is being held for questioning and is suspected of having been in contact with the attacker on the eve of the attack, the source said.
The attacker himself was taken into custody after police shot him 14 times after arriving on the scene.
His life was still in danger as of late Thursday, according to France's chief anti-terrorism prosecutor.
President Emmanuel Macron was due to hold a meeting of France's national defence council early Friday in the aftermath of what he described as a "terrorist, Islamist attack."
The attack came two weeks after a suspected Islamist beheaded a schoolteacher in a Paris suburb who had shown cartoons of the prophet Mohammed in a lesson on freedom of expression.
The French government's plans for a broad crackdown on Islamism and its fierce defence of the freedom to engage in blasphemy have sparked an angry reaction in many Muslim countries.