Story highlights Spanish police say they thwarted an attempt by the Sinaloa cartel to establish a foothold in Europe

"They're already operating in Europe," says author Malcolm Beith about the Sinaloa cartel

Four Mexican men are arrested

One of them is a cousin of Sinaloa cartel leader "El Chapo" Guzman

Spanish National Police said Friday they have seized hundreds of kilograms of cocaine in an operation that they said thwarted an attempt by Mexican drug dealers allied with the Sinaloa drug cartel to establish a foothold in Europe.

But an expert on the Mexican drug war, Malcolm Beith, disputed that assertion.

"They're already operating in Europe," he told CNN in a telephone interview, citing reports of the cartel shipping drugs to ports in England, Netherlands and Germany.

Spain is the principal gateway for drugs coming to Europe, Beith said.

The operation, dubbed "Dark Waters" and carried out in collaboration with the FBI, resulted in the arrests in Madrid of four members of the cartel, which is run by Joaquin Guzman Loera, also known as "El Chapo," police said in a statement.

Dark Waters was initiated in May 2009 by the FBI's office in Boston, Massachusetts, which was looking into the cartel, said the statement.

In October 2010, agents from the FBI met with the National Police's Central Brigade of Organized Crime after learning of the cartel's alleged plans to expand operations in Europe, it said.

The bulk of the investigation unfolded in the United States, where U.S. agents discovered that cartel members were planning to travel to Spain, a trip that occurred the following March, the statement said.

Once the suspects were inside Spain, they were placed under tight surveillance by Spanish police, it said.

"Thanks to the exchange of information with the FBI, one knew that the suspects planned to initiate important shipments of cocaine by ship, hidden in containers with legal, declared cargo," it said. "They adopted great measures of security to ensure the success of the operations, and sent various containers without any type of drug. Finally, they sent their first shipment in a boat from Brazil. The container, which was intercepted in late July in the Port of Algeciras, concealed 373 kilos of cocaine."

Investigators identified Jesus Gonzalo Palazuelos Soto, who moved to Madrid to oversee the operation, as a member of the group, it said. A few days later, he was joined by Jesus Gutierrez Guzman, a cousin of "El Chapo" and considered the leader of the group; Samuel Zazueta Valenzuela, a principal collaborator, and Rafael Humberto Celaya Valenzuela, who was overseeing the organization's "legal" affairs, it said.

The four men, aged 37 to 52, were arrested near their hotels in Madrid, it said. They had been sought by U.S. authorities for drug trafficking, money laundering and other charges.

Their hotel rooms contained $5,500, about 3,000 euros and 4,000 Mexican pesos, it said.

If indeed "Chapo" Guzman's cousin was among the men arrested, "that is a big deal," said Beith, whose books "The Last Narco" and "Hasta El Ultimo Dia," focus on the Mexican drug war. Though he said he had never heard of the man, "as a relative, he's clearly an envoy of sorts. He's not just some low-level conejo -- rabbit scout."

Beith said Friday's announcement was a testament to the improvement in intelligence sharing by international agencies. "As the drug wars come under fire from many quarters, one of the big advances that has been made has been intelligence sharing between U.S. agencies and their foreign counterparts, and this is further proof of that."