French investigators arrested two more people Sunday as they pieced together details about the motives and planning of the Tunisian who rammed a truck into a crowd in an ISIL-claimed attack that killed 84.

French investigators arrested two more people Sunday as they pieced together details about the motives and planning of the Tunisian who rammed a truck into a crowd in an ISIL-claimed attack that killed 84.

Mohamed Lahouaiej-Bouhlel staked out the Nice promenade with his rented truck twice in the two days before he smashed the vehicle into a crowd of people watching Bastille Day fireworks on Thursday night, according to a source close to the probe.

Mangled bodies were left strewn across the Riviera city's seafront in the grisly attack by a man described by those who knew him as a loner with tendencies towards violence and depression.

While some family and friends have described the 31-year-old as someone who smoked, drank and never attended the local mosque, others questioned indicated "a recent shift to radical Islam", said a police source.

However there has been no evidence yet linking him to the ISIL group, which on Saturday claimed the attack.

In Nice, many people were still desperately waiting for news of their loved ones among the dead.

Prosecutors said only 35 victims have been officially identified as they take painstaking measures to avoid errors of identification seen during the November Paris attacks.

"We have no news, neither good nor bad," said Johanna, a Lithuanian who was looking for her two friends, aged 20.

At least 10 children were among the dead as well as tourists from the United States, Ukraine, Switzerland Germany and about 10 from Russia, a local Russian association said.

Health Minister Marisol Touraine said that 85 people were still hospitalized, 18 people of them in critical condition.

On the second day of national mourning, the Russian Orthodox Church in Nice held an emotional mass for the victims. Another service was planned later at Paris's iconic Notre-Dame cathedral.