French authorities are determining whether a Tunisian who killed at least 84 people by ploughing a truck into Bastille Day crowds acted alone, but say the attack bore the hallmarks of Islamist militants.
Thursday night's attack in the Riviera city of Nice plunged France again into grief and fear just eight months after gunmen killed 130 people in Paris.
Those attacks, and one in Brussels four months ago, have shocked Western Europe, already anxious over security challenges from mass immigration, open borders and pockets of Islamist radicalism.
The truck zigzagged along the city's seafront Promenade des Anglais as a fireworks display marking the French national day ended.
At least 10 children were among the dead. Of the scores of injured, 25 were on life support, authorities said.
The driver, 31-year-old Mohamed Lahouaiej Bouhlel, shot dead by officers at the scene, was known to police for petty crimes but was not on a watch list of suspected militants.
The investigation "will try to determine whether he benefited from accomplices," Paris prosecutor Francois Molins said.
"Although yesterday's attack has not been claimed, this sort of thing fits in perfectly with calls for murder from such terrorist organisations."
Bouhlel's ex-wife was in police custody, Molins said.
Dawn broke on Friday with pavements smeared with dried blood. Smashed children's strollers, an uneaten baguette and other debris were strewn about the promenade. Small areas were screened off and what appeared to be bodies covered in blankets were visible through the gaps.
The truck was still where it had come to rest, its windscreen riddled with bullets.
Tunisian security sources told Reuters the suspect had last visited his hometown of Msaken four years ago.
He had three children and was not known by the Tunisian authorities to hold radical or Islamist views.
"France is filled with sadness by this new tragedy," President Francois Hollande said in a dawn address.
A state of emergency imposed after the November attacks was extended by a further three months.
Military and police reservists would be called up to help enforce it.
Police carried out a controlled explosion on a white van near the home, blowing the doors open and leaving shattered glass all around, but it was not clear whether they found anything incriminating.
With presidential and parliamentary elections less than a year away, French opposition politicians seized on what they described as security failings that made it possible for the truck to career 2km through large crowds before it was finally halted.
Christian Ertosi, a security hardliner who was mayor of Nice until last month and is now president of the Riviera region in which Nice lies, had written on the eve of the attack to Hollande to demand more funding for police.
On social media, Islamic State supporters celebrated the high death toll and posted a series of images, one showing a beach purporting to be that of Nice with white stones arranged to read "IS is here to stay" in Arabic.