Three Moroccans who survived a migrant boat accident are accused of organising the journey from North Africa and have been charged with manslaughter.
At least eight African immigrants died when the boat, thought to be trafficking some 30 people, hit rocks near Cadiz in southern Spain.
Fifteen people survived the accident, which occurred on Monday.
Some of them told the police that the suspects were in control of the vessel when they embarked in Morocco.
Once they were close to the Spanish coast the people traffickers pushed some of the migrants off the boat, causing it to capsize, said the survivors, according to officials.
The Spanish authorities said the search for survivors and bodies continued at sea and on land.
Thousands of Africans risk their lives every year crossing the waters of the Mediterranean Sea at the Strait of Gibraltar and the Atlantic waters between the Canaries and Africa in order to reach Spain, where they either find work or remain temporarily before moving on to other European countries.
To make the sea voyage, African migrants often pay people-smugglers to take them to Spanish territory in fragile and often overloaded boats.