The 29-year-old Frenchman was identified from a finger found at the site of the Bataclan massacre, Agence France-Presse reports.
This apparently matched fingerprints on police files. Mostefai had a criminal record, but reportedly had not served time in jail.
The brother and father of Mostefai have been taken into police custody, and their homes have been searched.
Mostefai’s older brother reportedly told AFP before going to the police station that he had not had contact with his younger brother for several years:
It’s crazy, insane. I was in Paris myself last night, I saw what a mess it was.