The gunman, identified as Radouane Lakdim, 25, who witnesses said claimed to be acting on behalf of the Islamic State, was later killed by police officers who stormed the market. Colonel Beltrame, who the authorities said had voluntarily exchanged himself for a hostage, was wounded in the exchange of fire.

The attack rattled nerves in a country that has been hit hard by terrorism in recent years, and it underscored the threat posed by individuals inspired by terrorist propaganda but who act outside of any structured networks, making it difficult for intelligence services to monitor them.

“The level of the terrorist threat on our territory has not waned,” said François Molins, the Paris prosecutor, who handles terrorism investigations nationwide. “It is the result of radicalized individuals who are on our national territory.”

Whether or not Mr. Lakdim, the attacker, had any direct contact with the Islamic State remained unclear. He was born in Morocco but lived in Carcassonne, about 60 miles southeast of Toulouse, and was known to the police as a petty criminal and drug dealer. He had previous convictions for illegally carrying a firearm and possessing drugs.

Mr. Molins said that Mr. Lakdim had been flagged by French intelligence services in 2014 “because of his radicalization and of his ties to the Salafist movement,” and that he had been under surveillance in 2016 and 2017.