Spanish police have opened an inquiry after four people were killed as a result of being forced out of a moving speedboat into the sea just metres from the southern shores of Andalucía. In a tactic the EU border agency has said is becoming increasingly common, the people who died on Wednesday were among 27 passengers apparently forced off the fast-moving vessel by its drivers near Camposoto beach in the province of Cádiz, officials said. Another eight were left near Sancti Petri beach. All 35 people were believed to be migrants from northern Africa, the Spanish government press office said. The rest of the passengers forced off the boat, including six children, survived. Four people were taken to hospital, some with hypothermia. Images on social media appeared to show a black vessel near a beach as people were being pushed off the side of the boat. A handful of others were already in the cold, choppy waters, struggling to stay afloat. The incident comes months after an internal report by Frontex, the EU border and coast guard agency, said smugglers were increasingly using speedboats to transport migrants from Morocco to Spain. Often when they neared the shores of Europe, the drivers used violence to hurriedly throw or force migrants off the boats, turning back quickly to avoid being intercepted by police, the newspaper El País reported. Witness accounts suggested this tactic was used when the boat neared the shores of Cádiz on Wednesday. Javier González, who manages a company offering windsurfing lessons, told reporters: “We saw a drug trafficking boat arriving but they weren’t trafficking drugs but migrants. Suddenly, they began jumping and some were thrown.” He said that as he and others scrambled to help those in the water, the boat sped away. González and his son rescued eight people, he said. “One of them told us that they put a gun to him and told him that either he jumped or they would shoot him.” Using his company’s inflatable dinghy, González and his son made two trips to pull people from the water. “They were all young, between 15 and 20 years old. Those who were able to talk thanked us,” he said. “You can’t imagine their faces, they were terrified. They said they had paid €5,000 (£4,300) for this trip.” Police said they were investigating the incident and had launched a search for the boat, described as the type of high-speed vessel normally used to traffic drugs between Spain and Morocco. Those on the frontlines of the drug battle said they were not surprised by the shift in strategy. “They are unscrupulous people and, when you don’t have scruples, it doesn’t matter if what they have to throw overboard are bales of hashish or people,” Francisco Mena, who leads a Cádiz-based anti-drug organisation, told the news agency EFE. The deaths – coming as Spain grapples with a 70% increase this year in irregular migrant arrivals – prompted a call by Spain’s second deputy prime minister, Yolanda Díaz, for Europe to change its approach to migration. “Horrified by this news,” she wrote on social media. “Something has gone very wrong when those who come in search of a better life only find death … We need a Europe that welcomes and is inclusive so that our seas and oceans stop being a mass grave.”