An estimated 60,000 people from Morocco crossed into Spain’s North African enclave of Ceuta within 24 hours, in an unprecedented surge that left dozens dead. The crossings underline Ceuta’s continuing role as a gateway to Europe and have prompted questions about what caused Moroccan authorities to allow or fail to prevent such a large movement.

Ceuta has about 83,000 residents and has long been a focal point for migrants seeking to enter Europe. By Friday evening, Spanish officials said more than 48,000 of those who crossed had voluntarily returned to Morocco; the BBC report examines the possible trigger for the surge and what may happen next.