Investigators probing the random killing of three people in the town of Istres, near Marseille, believe the suspected killer bought the Kalashnikov rifle he used in the shooting over the internet, a report in the French media claimed on Friday.

One of the victims, a man thought to be in his sixties, was shot dead at the wheel of his car, the other two were male pedestrians, both local men aged 35 and 45 respectively.

A witness to the incident described how the 19-year-old suspect had walked around with his rifle poised "as if he was out hunting" and might easily have killed more people.

The suspect was arrested not far from the scene and remains in police custody. French radio RTL reported on Friday that a source close to the investigation had suggested that he had bought the military style weapon online.

If this is confirmed it will put more pressure on France’s Interior Minister Manuel Valls, who visited the scene of the shooting on Thursday. Valls has vowed to crack down on the growing problem of gun crime in France.

"It is clearly very worrying. It once again raises the problem of the number of weapons in circulation," Valls said on Thursday

What pushed the teenage suspect to take the rifle into the street and murder three innocent people remains unclear.

Immediately after the shooting it was reported that the attacker had claimed he belonged to Al Qaeda, but it seems police have no proof to suggest this was the case.

“He has not given us any rational explanation and had no link at all with the victims,” a police source told RTL radio.

According to Europe1 radio the shooter's Facebook page reveals he had a love of guns and was convinced Al-Qaeda inspired Toulouse gunman Mohamed Merah was innocent.

Investigators are looking into his background to probe whether he had a history of mental health problems.

A former class mate described the teenage gunman as "twisted", who had a love of watching gory films, according to Europe1.