The main suspect in the bombing last week in the central French city of Lyon has told investigators that he had pledged allegiance to the Islamic State, a French judicial official said on Thursday.
The official, who was not authorized to speak publicly about the case, identified the suspect as Mohamed M., but did not provide any additional details.
The suspect was arrested on Monday at a bus stop in Lyon and is still in custody, where he admitted to investigators that he had carried out the bombing. But his exact motivations — and whether he had direct ties to the Islamic State or to a wider network — remain unclear, the French authorities said.
There has been no claim of responsibility from the Islamic State for the bombing, in which 13 people were lightly wounded by a device packed with screws, metal balls and batteries that exploded outside a bakery in the center of Lyon, France’s third-most populated city.
Over 300 police officers used witness accounts and footage from video surveillance cameras to retrace the suspect’s steps and identify him.
The French police had taken in the suspect’s parents and brother for questioning, but the Paris prosecutor’s office, which handles terrorism investigations nationwide, said on Thursday that all three had been released for lack of incriminating evidence.
Christophe Castaner, France’s interior minister, told the CNews television channel on Tuesday that the suspect was not previously known by French security or intelligence services.
Mr. Castaner declined to confirm or deny French news reports that the suspect was a 24-year-old Algerian, but he said the man had previously stayed in France on short-term visas and was then refused a student visa.
The bomb showed “very efficient” technical know-how but had a “very small volume of explosives,” Mr. Castaner said.
Gérard Collomb, the mayor of Lyon, told Franceinfo on Wednesday that the suspect did not show any “exterior” signs of recent radicalization but that he had ordered materials online that could be used for homemade bombs.
France has been a target of terrorist plots in recent years, especially in 2015 and 2016, when Islamist extremists waged deadly, large-scale attacks. In the past few years, attacks have been rarer and much smaller in scale. In December, in the eastern city of Strasbourg, five people were killed by a gunman at a Christmas market.