No group claimed responsibility for Thursday night's slaughter of tourists and locals packing the upscale seafront, where an estimated 30,000 had just watched a Bastille Day fireworks show.
French Interior Minister Bernard Cazeneuve, asked on Friday if he could confirm the attacker's motives were linked to jihadism, said: "No". The driver's estranged wife is also being held by police.
Police said those arrested were all males and not related to the driver of the truck, 31-year old Mohamed Lahouaiej-Bouhlel but were friends of the Tunisian-born migrant.
Police found two automatic weapons, ammunition, a mobile phone and documents in the truck, as well as fake weapons at his home.
"I went to bed and started to hear people screaming and I saw people running, running, running, police, people crying, people screaming", she said.
Bouhlel, however, was not someone suspected by French or Tunisian authorities as having been radicalised, officials in both countries said.
Detectives and forensics experts entered his apartment in a working-class district of Nice with an armed police unit in support around 9:30am (0730 GMT) and brought out bags of material later.
According to some reports, the attacker spent the nine hours in his truck waiting for the revelers to turn out for the evening before plowing his vehicle into the crowd.
One of the men being held was arrested Friday and three others on Saturday morning, a police source added.
President Francois Hollande said 50 people were "between life and death", while several people were among the missing and a "small number" of Britons were injured.
French President Francois Hollande flew to Nice, met the wounded and said the terrorist struck the crowd "merely to satisfy the cruelty of an individual - and maybe a group".
Prosecutors say the probe will be handled by anti-terrorism investigators.
A French parliamentary inquiry last week criticised numerous failings by the intelligence services over the Paris attacks. Army reservists will be called up to boost security.
"He carried out the operation in response to calls to target nationals of states that are part of the coalition fighting Islamic State".
The first driver shouted "Allahu Akbar" (God is greatest) as he drove into people in the eastern city of Dijon, injuring 13.
Concerns had been raised about a Galway man believed to have been injured in the Bastille day incident - when a man drove a lorry through a crowd of thousands of people.
The man committed suicide in his prison cell in 2016 while awaiting trial.
Stephanie Simpson, from the Lenval children's hospital in Nice, said five children remained in critical condition, one was in a "very bad" condition, three were on artificial respiration, one had been stabilised and one eight-year-old child remained unidentified.
"There are French among the victims and also many foreigners from every continent and many children, young children", Hollande said after visiting a hospital.
Two US citizens were confirmed dead by the State Department.
The father confirmed that his son was "not on good terms" with his wife.