French Interior Minister Claude Gueant on Tuesday said the gunman who killed four people outside a Jewish school in the southwestern city of Toulouse has not been identified yet, adding that more hard work was required to arrest the "cruel" killer.

"For now, the work continues... If he would have been identified we would have significant progress. Unfortunately, we are not there," Gueant said.

Speaking to local broadcaster Europe 1, Gueant said the killer of "extreme coldness" and "great cruelty" had a "recording camera which is placed on the chest and adjusted by straps."

"An investigation like this required much work... I'm sure we will end (it) but we still need to work. Each hour of work provides new elements," he added.

A gunman shot dead four people, including 30-year-old Jonathan Sandler, his two sons Gabriel and Arie, and seven-year-old Myriam Monsonego, on Monday morning outside a Jewish school in Toulouse, before fleeing on a scooter.

All French schools on Tuesday observed a minute of silence in memory of the four, with French President Nicolas Sarkozy paying his respect at a secondary school in Paris.

Sarkozy has confirmed earlier that the bullets fired at Monday's school attack site had been fired from the same weapon used in similar shootings on March 11 and 15, which left three French soldiers from ethnic minorities dead.

The president announced on Monday night to raise the terror alert to the highest level in southwestern France.