Five lifeguards from Spain have been arrested in Greece on suspicion of people trafficking refugees.
"A preliminary investigation has been opened," a Greek coastguard spokeswoman said, adding that the suspects were likely to be charged with "attempting to facilitate the entry of illegal migrants."
Another coastguard source identified the suspects as Spanish.
The group claimed they were working on the island of Lesbos for the Basque lifeguard group DYA.
The DYA is among dozens of non-governmental groups working there to help refugees and migrants arriving from the Turkish coast.
Last year some 850,000 people, predominately from Syria, attempted the risky crossing of the Aegean Sea to reach Europe, with at least 800 of them perishing.
Two of the suspects were arrested on board a speedboat where coastguards found an unregistered wireless radio which was capable of tapping into official frequencies.
Greek authorities have recently begun to give greater scrutiny of the aid agencies on the ground. They have been allowed to operate freely in the distressed region, but this has often resulted in problems coordinating rescues.
