PARIS (AP) — The prosecutor’s office for the Paris suburb of Saint-Denis said that some 50 young people have been detained for allegedly throwing stones at police cars, setting fires and lobbing smoke bombs into a school.

Police rounded up the youths on Tuesday morning and were questioning them.

The prosecutor’s office says investigators must ascertain whether each of those arrested was involved in the unrest, which began at the school. The group can be held for 24 hours.

The prosecutor’s office says it is unusual to detain such a large number of suspects at once. There was no clear information about a possible motive.

One hypothesis linked the turmoil to ongoing outrage over the Feb. 2 assault of a young man who was allegedly raped with a police baton.