Eyewitnesses in Nice have said that they assumed the driver of the lorry which drove into a crowd, killing 84 people who were celebrating Bastille Day last night, had lost control as panic broke out.
The driver is believed to be a 31-year-old French-Tunisian man, who was known to police - but for petty crime, not terrorism, according to reports in France.
Interior Minister Bernard Cazeneuve, said identification papers were found in the truck and that investigators were trying to determine whether they were legitimate.
Witness Eric Ciotti told Europe 1 radio of the horrifying moments people fled to safety - with some running into the sea in a desperate attempt to flee.
He added: "A person jumped on to the truck to try to stop it.
"It's at that moment that the police were able to neutralise this terrorist. I won't forget the look of this policewoman who intercepted the killer."
Egyptian Nader El Shafei told the BBC he saw the driver "looking very nervous".
Eyewitnesses said the driver swerved from side to side to kill as many people as possible.
He said: "I kept yelling at him, waving with my hands to stop and trying to tell him that there is a lot people under his truck - dead already.
"But he did not give any attention to anyone outside the truck.
"And suddenly I saw him picking up something like a cellphone. I thought he would call the ambulance for the accident but it seemed that I was wrong, because he just picked up his gun and he started to shoot the police.
"Just when they (the police) arrived they just felt something was wrong so they kept yelling at him and when he did not step out - they saw him from the window taking his gun out.
"They knew that would be a gun shooting so they just killed him right away - they did not wait to negotiate or something, they just opened fire on him."