On Friday, Lakdim carried out the attacks in Carcassonne and Trebes after dropping his younger sister at school in the morning, Molins said.

The Islamic State group claimed that the attacker was answering its call to target nations in the US-led coalition fighting IS militants in Syria and Iraq.

A police officer, Lieutenant Colonel Arnaud Beltrame, was killed after swapping himself with a hostage being held by Lakdim in the supermarket.

Three people, including Beltrame, were killed in the supermarket in Trebes while a fourth person was killed earlier during a carjacking in the nearby city of Carcassonne.

When Beltrame entered the supermarket during the hostage situation, he started a negotiation with Lakdim, who had found shelter in the store's vault with the hostage.

"He [Beltrame] raised his hands in the air, dropped his weapon and asked to take the place of the person taken hostage," Molins said.

A few minutes after Beltrame entered the vault and offered himself as a hostage, Lakdim released the woman he was holding.

Beltrame then contacted his colleagues to ask them to leave the premises. He added the attacker was demanding the release of Salah Abdeslam, the sole surviving assailant of the November 13, 2015, attacks in Paris that left 130 people dead.

Beltrame surreptitiously left his mobile phone on so that police outside could hear what was going on inside the supermarket. Molins said that several explosive devices were placed around the supermarket by Lakdim and the attacker threatened to set them off.

Elite police units stormed the store after hearing gunshots. They killed Lakdim and found their Beltrame seriously wounded.

Beltrame died in a local hospital on Saturday from several stab wounds "including a very severe injury in the trachea and larynx," Molins said.

The prosecutor said Lakdim arrived in France with his family in 1992 soon after he was born in Morocco.

Both a French and Moroccan national at first, he was then naturalised as a French citizen in 2004 when he was 12, following his father's naturalisation.

Molins said Lakdim had been listed on a police register for radicalised people since 2014 because of its suspected links with the local Salafist circles. He was still under an ongoing and effective intelligence monitoring in the days leading up to the attacks, the prosecutor said.

But the surveillance didn't allow police to detect any warning signs that he was going to commit a violent action or any intention to go to Iraq or Syria, Molins said. When he was jailed in 2016, the prison administration noticed no visible sign of radialisation.

In addition to the girlfriend, a 17-year-old male has also been in custody since Friday night. The second suspect was a "very good friend" of Lakdim and was under his religious influence, a witness told investigators.

He also denied any involvement in the preparation of the attacks during his questioning, Molins said.

A national memorial ceremony for Beltrame will be held Wednesday in the main courtyard of the Invalides monument, home to Napoleon's tomb, in the presence of Beltrame's family and the families of other victims of the attack.

French President Emmanuel Macron will attend and address the event.

AP