Some of them were slaves that converted to Islam but most had probably never been slaves and had come to North Africa looking for opportunity.
وكان بعضهم من العبيد الذين اعتنقوا الإسلام، لكن ربما لم يكن معظمهم عبيدًا وقد جاءوا إلى شمال إفريقيا بحثًا عن فرصة.