A 21-year-old Swedish man of Somali origin, arrested by the Guardia Civil in Barcelona last week, was planning a murder on the Costa del Sol, according to the investigation.
The man is thought to be one of the leaders of a dangerous gang of drug traffickers, money launderers and assassins, known as the "death patrol", and is wanted in Sweden in connection with seven killings in Stockholm.
The Spanish investigation, in collaboration with the Swedish authorities, began in September in Malaga province, where the suspect had been hiding.
The Swedish authorities, who had arrested two other leaders of the organisation in Sweden, warned of an imminent assassination the gang was planning to carry out somewhere in the province of Malaga.
This caused the Guardia Civil to speed up their investigation and carry out the arrest while the suspect was on a trip to Barcelona.
The Spanish authorities are now looking back over unsolved crimes in this country in case they find any connection with this group.