An eyewitness to the attack Thursday night in Nice, France, that killed at least 84 people and injured more than 100 described the man linked to the incident as a "young guy, very focused on what he was doing."

French police and prosecutors have confirmed that he was 31-year-old Mohamed Lahouaiej Bouhlel, a French resident born in Tunisia with a record of petty crime.

According to officials, Lahouaiej Bouhlel drove a multi-tonne truck for about two kilometres through a packed crowd celebrating Bastille Day on the coastal city's famed seaside promenade. The police officers who eventually killed him in a shootout said he also had fired a handgun at bystanders during the attack.

A top French prosecutor said Lahouaiej Bouhlel appears to have obtained the truck yesterday morning, but investigators don't know if it was rented, stolen, or if he potentially owned the vehicle.

Speaking to reporters Friday, French President François Hollande described what happened as a "terrorist act."

He was 'frankly, not normal'

Prosecutor Francois Molins said that Lahouaiej Bouhlel was known to authorities for matters of threats, violence, theft and damages committed between 2010 and 2016. He had one criminal conviction, for road rage, and was sentenced to probation three months ago for throwing a wooden pallet at another driver.

But Molins said "he was on the other hand totally unknown to intelligence services ... and was never placed on a watch list for radicalization."

Lahouaiej Bouhlel was named as the driver after police found an identification card, possibly a driver's licence, in the cab of the delivery truck used in the attack. His identity was later confirmed through fingerprints.

The police investigation "will try to determine whether he benefited from accomplices," Molins said. "It will also try to find out whether Mohamed Laouaiej Bouhlel had ties to Islamist terrorist organizations."

Lahouaiej Bouhlel was married with three young children, though he and his wife have been estranged for several years and were in the process of divorcing. In 2012, he was barred from entering their former family home following accusations of domestic abuse.

His wife is in police custody, Molins said.

Woman who says she is the neighbour of the Nice attack suspect describes him 1:00

The Associated Press reported that Friday afternoon police raided a flat in a mid-rise building about 15 minutes drive from the promenade, where Lahouaiej Bouhlel seemingly lived alone. His wife and children live in a nearby apartment.

According to CNN, a police forensics team spent nearly four hours searching a small delivery van — not the one used in the attack — that was parked outside the building where he resided. It reportedly belonged to Lahouaiej Bouhlel.

A neighbour, who wished to remain unnamed, described him as, "frankly, not normal," adding that he was "almost always alone," would never engage with people in the apartment building and that he had a tendency to stare at children.

"He was frightening," she said.