Spain's interior ministry has arrested four members of a major Mexican drug-trafficking cartel, including the cousin of Mexico's most wanted man.
The four men, who belong to Sinaloa cartel, one of the biggest criminal organisations in the world, were arrested near their hotels in the capital Madrid, the ministry said in a statement on Friday.
The statement gave the names of the men as Jesus Gutierrez Guzman, Rafael Humberto Celaya Valenzuela, Samuel Zazueta Valenzuela and Jesus Gonzalo Palazuelos Soto.
But the statement did not say precisely when the arrests were made, and when called by phone ministry officials could not immediately give exact details of the dates.
The ministry said the cartel wanted to make Spain a gateway for operations in Europe, even carrying out test runs using shipping containers without drugs, said the Associated Press news agency.
But investigators managed to monitor many of the group's activities and intercepted a container carrying 373kg of cocaine in late July before moving in to make the arrests.
"Our country was going to be used as a point of entry for large shipments of narcotics," the ministry said in a statement.
Jesus Gutierrez Guzman is alleged to be the cousin of Joaquin Archivaldo Guzman, known as El Chapo, the leader of the cartel and Mexico's most wanted man.
Since escaping prison in 2001, Joaquin Archivaldo Guzman has run the Sinaloa cartel, one of Mexico's two most powerful drug-organisations, from a series of hideouts and safe houses across Mexico.
The joint operation against the cartel, dubbed Dark Waters, began in 2009. Officials said it was made possible thanks to agents using "the most modern research techniques", which had at all times been supervised by judges and prosecutors.
The ministry's statement said "the bulk" of the investigation was carried out in the United States.
The news caused a splash across Mexican media when it emerged that one of the suspects was linked to Mexico's Institutional Revolutionary Party or PRI, whose candidate won July's presidential election.
Rafael Celaya had applied to be a PRI candidate for federal deputy for the northern state of Sonora in the July poll, but was unsuccessful, according to the official party website.
A photo on Celaya's Facebook page also showed him standing next to incoming president Enrique Pena Nieto at a campaign event.
A PRI official said the party was looking into the matter and would release a statement on it later.
The Sinaloa cartel and its allies have been waging a brutal war against the paramilitary Zetas cartel across Mexico, often carrying out mass killings that have left hundreds of dismembered bodies dumped in public places.