Police questioned and then released relatives of a man shot dead at a Paris airport, as investigators sought clues on why he attacked an army patrol in an incident that has pushed security to the forefront of France's election campaign.

Paris prosecutor Francois Molins said late on Saturday that the man, named as 39-year-old Ziyed Ben Belgacem, had shouted he was there to "die for Allah" when he tried to seize a gun from a woman air force member on patrol at Orly airport.

After throwing down a bag containing a can of petrol and putting an air pistol to the head of the soldier, he was shot three times by her colleagues.

More than 230 people have died in France in the past twoyears at the hands of attackers allied to the militant Islamistgroup Islamic State, making security a key issue in the two-round presidential election on April 23 and May 7.

Emmanuel Macron, the centrist frontrunner, told France 2 television on Sunday it was "essential" to maintain the current state of emergency, in force since November 2015. He reiterated that he would strengthen military operations and intelligence against Islamist militants.

Conservative Francois Fillon said France was in a "situation of virtual civil war" and spoke out against the idea of lifting the state of emergency, as floated by the justice minister last week.

Far-right candidate Marine Le Pen, running on an

anti-immigration, anti-European Union ticket, told a rally that the government was "overwhelmed, stunned, paralysed like a rabbit in the headlights."

IN AND OUT OF PRISON

Belgacem, who had been in and out of prison for theft and drug offences according to judicial sources, was already on the authorities' radar. They said he became a radicalised Muslim when he served a prison term several years ago for drug-trafficking.

He had been reporting regularly to police under the terms of a provisional release from custody, and did not have the right to leave the country.

Several hours before he was killed, Belgacem had shot and wounded a police officer with his air pistol when he was stopped for a routine traffic check north of Paris, officials said. He then fled the scene.

He later entered a bar where he was a regular customer in Vitry-sur-Seine on the other side of Paris, and opened fire with his air gun without hitting anyone. He also stole a car before arriving at the airport.

Belgacem's father, who was initially detained by police but released late on Saturday, denied his son had been involved in terrorism.

"My son has never been a terrorist. He has never prayed: he drinks. And, under the influence of alcohol and cannabis, this is what happens," the father, whose name was not given, told Europe 1 radio. 

He said he had received a phone call from his son in which Belgacem referred to shooting the police officer, saying: "I ask your forgiveness. I screwed up with a policeman."

An autopsy showed the presence of cannabis and cocaine in Belgacem's body, as well as an alcohol blood level of 0.93 grams per litre, a judicial source said on Sunday.

The legal limit for alcohol while driving in France is 0.5 g/l.

An earlier search of Belgacem's home had found several grams of cocaine, the prosecutor said.

Belgacem was born in Paris, according to the prosecutor. French media said his family was of Tunisian origin.

A brother and cousin of Belgacem were also questioned by police and then released on Sunday, the judicial source said.