Survivors told police that some of their fellow passengers began “attacking dozens of people, beating and abusing them in various ways”, a police statement said.
“In some cases, they threw migrants into the sea alive and refused to rescue those who fell in by accident.”
Suspected killings were also documented “simply because some people protested or expressed their dissatisfaction with the conditions of the voyage”, the statement said.
One male passenger died in hospital after being found seriously ill when the vessel was intercepted, according to police.
Those on board included people from Senegal, Guinea, Mali, Gambia, Sierra Leone, and Guinea-Bissau, local media reported.
All 19 suspects are in pretrial detention facing charges of facilitating irregular immigration, homicide, assault and torture.
Spain is one of the frontline entry points for irregular migration into Europe, with most entering via the Canary Islands.
Almost 47,000 migrants reached the archipelago in 2024, setting a record for a second year, but numbers have been far lower so far this year, the Spanish Interior Ministry said.