Leading suspect Yassine Salhi as well as his sister and wife remained in custody in the city of Lyon a day after he allegedly crashed a truck into a U.S.-owned chemical warehouse and hung his employer's severed head on a factory gate, officials said.
A fourth person who had been detained Friday was released, said Paris prosecutor's office spokeswoman Agnes Thibault-Lecuivre. Under French anti-terrorism laws, Salhi and the two women can be held up to four days before either being released or handed preliminary charges and locked up.
Investigators have not turned up any motive or possible foreign connection, Thibault-Lecuivre said.
No group immediately claimed responsibility. The severed head appeared to mimic the Islamic State group's practice of beheading prisoners and displaying their heads for all to see, and came days after the militants urged attacks during the Muslim holy month of Ramadan. French authorities say Salhi had links to radical Salafists in the past.
Separately Saturday, hundreds of people turned out in the region to honor slain businessman Herve Cornara and denounce the violence. Dozens turned out for a minute of silence in Saint-Quentin-Fallavier, the town southeast of Lyon where Friday's attack took place at an Air Products chemicals warehouse.
Several hundred people also gathered outside a housing project in the town of Fontaines-sur-Saone to honor Cornara, 54, the manager of a transportation company that had employed Salhi since March. They recalled a kind, humble man who was active in the community of the Lyon suburb.
"He lived on the fifth floor, me on the fourth. He spoke with all the young people in the neighborhood. He didn't differentiate between (non-Muslim) French and Muslims," said Leila Bouri, a 24-year-old cafeteria cashier. "If you ever had a problem, you would go see him."
"When I heard this, I was shocked. It's shameful," she said. "I am a Muslim, but you can't kill like this. It's not who we are. In Islam, we're not told to slit throats. We only slit the throats of sheep. You don't slit the throats of people."
The suspected killer, she added, "isn't a Muslim in my opinion."