NICE - The truck driver who caused carnage in the French city of Nice showed "recent interest" in jihadist activity, investigators said Monday, four days after a massacre that sparked fierce criticism of the government's security record.

Emotions are running high in France after the third major attack in 18 months.

Prime Minister Manuel Valls endured a humiliating chorus of boos and heckles at a ceremony of remembrance for the victims during a visit to the Riviera city Monday.

The Islamic State group claimed the attack by 31-year-old Mohamed Lahouaiej Bouhlel, who used a truck to mow down people leaving a Bastille Day fireworks display, killing 84 people.

Paris prosecutor Francois Molins said there was no evidence of Bouhlel's allegiance to IS but that a search of his computer "showed a clear, recent interest for the radical jihadist movement."

He also confirmed the attack was "premeditated".

In the two weeks prior to the attack Bouhlel carried out near-daily internet searches for IS propaganda videos and readings from the Koran, Molins said.

The father-of-three also searched for information about the terror attack on a gay nightclub in Orlando that left 49 dead, and the Paris suburb of Magnanville -- where a police couple were killed last month. Both attacks were linked to IS. His computer contained pictures of corpses and fighters posing with the IS flag.

The revelations came on the third day of mourning over the grisly attack.

A sea of people thronged the seafront in Nice for a solemn minute's silence. Similar ceremonies were held across the country, accompanied by the ringing of church bells.

In a sign of the mounting frustration over a string of attacks that have killed over 230 people since January 2015, Valls and two of his ministers were heckled by mourners who booed and shouted "Murderers" and "Resign!"

Valls said the "disgraceful" display reflected the "attitude of a minority" in the city, which is run by the opposition Republicans party.

The Nice attack came eight months after IS jihadists killed 130 people across Paris, and 18 months after three days of terror at the Charlie Hebdo weekly and a Jewish supermarket killed 17. Unlike the perpetrators of those attacks, Bouhlel, a petty criminal with a history of violence and depression, did not travel to the Middle East for training or jihad.

Molins said acquaintances described the Tunisian as "someone who did not practise the Muslim religion, ate pork, drank alcohol, took drugs and had an unbridled sexual activity".

About eight months ago ago, however, he snapped a picture of a news story headlined: "Man deliberately rams car into cafe terrace."

And earlier this month, he stopped shaving his beard.

On the day of the attack he appeared to have spent much of the national holiday on the promenade, taking selfies.

He also staked out the promenade in his truck twice before he struck, investigators said.

- 'Shameful' point-scoring -

While previous attacks in France spawned grand displays of national unity, there was no semblance of cohesion after the Nice massacre, which comes nine months ahead of presidential and legislative elections.

"Everything that should have been done in the past 18 months was not done," former president and opposition leader Nicolas Sarkozy, who is eyeing another run for the top job, said on Sunday.

Calling for all foreign-born terror suspects to be deported Sarkozy told TF1 television: "We are at war, outright war. So I will use strong words: it will be us or them."

Far-right National Front leader Marine Le Pen accused successive governments of the left and right of "grave mistakes" for "continuing to allow so many foreigners into France".

Interior Minister Bernard Cazeneuve pointed to a string of measures to fight terrorism, such as boosting the armed forces and adopting new anti-terror laws.

"There is no zero risk. By saying this we are telling the truth to the French and tackling the threat with lucidity," he said, accusing the opposition of using the attack for "shameful" point-scoring.

On Tuesday, parliament is to vote on extending the state of emergency imposed after the November attacks.

The government has also put out a call for volunteer reservists to supplement the strained security forces.

- Painstaking identification -

Six people were still being questioned over the attacks on Monday, including a 38-year-old Albanian suspected of providing Bouhlel with a pistol he used to fire at the police who shot him dead.

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

Molins said 71 of the 84 victims had been officially identified, with the authorities taking painstaking measures to avoid errors of identification seen during the Paris attacks last November.

At least 10 children were killed, along with tourists from Ukraine, Switzerland, Germany and several people from Russia and its neighbours.

Scores are still being treated in hospital, 19 of them in critical condition.