At least 50 migrants were tortured and thrown overboard after being accused of witchcraft on a boat crossing to Europe, police have said.

The first allegations of mass high seas executions on board the overcrowded vessel emerged at the start of the month after 248 survivors were rescued off the African coast and taken to Gran Canaria.

Detectives began investigating after witnesses brought ashore claimed to have seen fellow migrants beaten up and shot after people smugglers in charge of the boat accused them of being witches when they suffered engine problems and started running out of food.

Subsequent reports said 17 men – 16 Senegalese nationals and one Gambian – had been arrested and remanded in custody by a Spanish judge investigating the allegations.

On Wednesday police said 19 suspects had been detained as they released the first images of the arrests and detailed for the first time in an official statement the results of their inquiries into the nightmare Atlantic crossing so far.