PARIS — Several hundred police officers descended on northern Paris on Saturday, raiding homes and businesses and arresting people believed to have links to radical Islam as part of an investigation into the killing of a Muslim schoolteacher this week by two men who the police called terrorists.
The investigation was launched, along with the arrest of a 23-year-old man, just a day after the death of Mohammed Brahmi, who was found shot in his home on Thursday evening and who died later of his wounds. The French Interior Ministry identified the suspect as Karim Cheurfi, a 38-year-old gun collector who had an out-of-court decision pending related to a manslaughter charge.
It was Mr. Brahmi’s death, which police described as “unacceptable,” that led to the planned searches on Saturday and the announcement of a detentions order, as French authorities increasingly link the sudden attack on Mr. Brahmi to the larger attack that killed 85 people in Paris in November 2015.
But when it came to talking about Mr. Brahmi, authorities on Saturday spoke only about what they called an action by “an assailant armed with a big automatic pistol,” which went on for 10 minutes before his death.
Many feared that more than a gunshot could have led to his death. Analysts focused on the broader question of why the motives of an apparently normal 48-year-old teacher had devolved into gunplay.
“We cannot know what the killer considered to be at stake,” said Benjamin Griveaux, a government spokesman. “This is a tragedy that has stunned us.”
A joint statement of prosecutors and security officials said: “We consider it is necessary to determine quickly the furtive nature of his movements around Paris, before he committed the terrible act. There were traffic jams and pre-arranged meetings that might indicate his preparations.”
But officers said they were unsure whether Mr. Brahmi knew his attackers.
In addition to the man who was detained, the police searched for the man who left behind an Islamic prayer rug that Mr. Brahmi used in his house.
It was worn with a prayer of Allah, punctuated with Arabic calligraphy on the knee and matched the corresponding prayer rug found in a garbage can outside Mr. Brahmi’s apartment building on Saturday.
And it was “intercut into the pajamas that he wore when he died,” said Jean-Michel Simon, a security official.
A week ago, Mr. Brahmi’s colleagues arranged to send him home at 10:45 a.m. on Wednesday. He was shot shortly after 12 p.m.
Under French law, after the death of a suspected terrorist, there are three days for investigators to prepare an exhaustive, painstaking search for information, if any, on the motivation and “state of mind” of the man who committed the act. But it also gives them an opening to act.
Mr. Brahmi’s brother, Abdelkader, who was dismissed from the teachers’ union, said his family was “deeply shocked” and that it was difficult to comprehend his slaying. “We’re all completely shocked,” he said.
Authorities hoped to glean more information on Mr. Brahmi’s relationship with Mr. Cheurfi and from his residence where the two shared a third-floor apartment in a building near the Sainte-Foy train station. They also sought to question witnesses in his rented apartment, where anti-terrorism investigators last month searched for clues about the motivations of one of the attackers who killed 84 people and injured another 400 at a rock concert in Paris on Nov. 13, 2015.
Few people have come forward to speak publicly about Mr. Brahmi and police investigations into his death, which shook France and led to heightened security elsewhere in the country.
Among those who spoke out Saturday was Ms. Carla D’Auito, one of the men who found Mr. Brahmi’s body. The pair heard the gunshot, saw Mr. Brahmi fall to the ground and went to take him to an ambulance, she said, adding that “many people” around her also had doubts about Mr. Brahmi’s murder.