MADRID (AFP) - A Spanish customs agent died Sunday (July 11) after a customs agency helicopter crashed into the sea while chasing suspected drug smugglers off Spain's southern coast, officials said.

Two other customs agents who were also on board the chopper when it went down in waters near Sotogrande, a coastal town near Gibraltar, survived the crash, a union for Spain's Civil Guard police force said in a tweet.

The cause of the crash is under investigation, it added.

The helicopter will be brought to the surface so its black box can be retrieved to try to determine what caused the aircraft to go down, Budget Minister Maria Jesus Montero, whose ministry is in charge of customs, told reporters.

"It is too soon to suggest a hypothesis. All hypotheses are open," she said.

Spanish Prime Minister Pedro Sanchez offered his condolences to the agent's family, and thanked security forces for their work, in a tweet.

Gibraltar Chief Minister Fabian Picardo also expressed his condolences to the agency's family and sent his "best wishes for a speedy recovery" to the two survivors.

He said police forces in the tiny British territory on Spain's southern tip would continue to "work closely" with their Spanish counterparts to fight "the evil of drug trafficking".

Spain's southern coast is a key entry point for drugs bound for Europe due to its proximity to Morocco, a major hashish producer. Smugglers use speedboats to make runs from Morocco.