A source close to the investigation told AFP that Mohamed Lahouaiej-Bouhlel, a 31-year-old Tunisian, sent a text message just before the attack in which he "expresses satisfaction at having obtained a 7.65mm pistol and discusses the supply of other weapons".
He also took a selfie at the wheel of the 19-tonne truck in the days before he ploughed it into a crowd of people who had been enjoying a fireworks display on Bastille Day, France's national day, killing 84 and injuring about 300.
While some relatives and friends described the delivery driver as someone who drank heavily and never attended the local mosque, others questioned by investigators spoke of "a recent shift to radical Islam", said a police source.
But there has been no evidence yet linking him to the Islamic State group, which yesterday claimed the attack.
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An Albanian suspected of providing the driver with the pistol was arrested in Nice today. Lahouaiej-Bouhlel fired at police who sprayed his rampaging truck with gunfire, eventually killing him.
One of five other people being held over the carnage is a 22-year-old suspected of lending logistical support, said his lawyer Jean-Pascal Padovani.
The lawyer said the pair had only known each other for a few months.
Lahouaiej-Bouhlel's estranged wife, the mother of his three children, was released today after two days of questioning.
In Nice, many people were still desperately waiting for news of their loved ones.
"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 people from Russia, a local Russian association said.