A Spanish customs agent died Sunday 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 down 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.

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

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 agents who survived the crash.

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.