The aid workers -- two Danes working for the NGO Team Humanity, and three Spanish citizens with the organisation PROEM-AID -- were detained Thursday when coastguards patrolling off the island of Lesbos found two of them in possession of knives and an undeclared two-way radio.
The prosecutor in Lesbos' main town Mytilene on Thursday opened proceedings against the group on suspicion of "attempting to facilitate the entry of illegal migrants", and they were questioned over the weekend.
Greek authorities have in recent days begun a more stringent inspection of the aid groups operating on the holiday island -- which saw half a million migrants pass through last year on their way elsewhere in Europe -- in a bid to improve rescue coordination.
Seven volunteers -- including three Britons, two Americans, a Dane and a Cypriot -- had been arrested Saturday on suspicion of stealing lifejackets left behind by refugees.
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They were later released after convincing the authorities that they intended to use the lifejackets for makeshift bedding for the migrants.
"The work of volunteers on the Greek islands is of critical importance, and saves lives," he wrote on Facebook.
"Those crucial activities should be facilitated by the Greek authorities, and not impeded.