Italian police and Coast Guard officers seized the 'Iuventa' on the southern island of Lampedusa after the ship was earlier intercepted by the Coast Guard and escorted to the port.
According to an investigation begun in October last year, the Jugend Rettet vessel had been "regularly" picking up migrants in the Mediterranean close to Libyan waters and transferring them to other ships in international waters.
"The rescue - that is to say the transfer of migrants - took place when they were not in any specific danger," Ambrogio Cartosio, deputy prosecutor in the Sicilian port city of Trapani, told reporters.
But he stressed investigators had found no evidence of any "prearranged plan" between Libyan people traffickers and the NGO.
"I believe the Jugend Rettet crew is motivated by humanitarian aims," he said.
Jugend Rettet this week refused to sign the Italian government's new controversial rules for NGOs rescuing migrants in the Mediterranean, but Ambrogio denied this was linked to the Iuventa's seizure.
"The fact that it didn't sign the accord (code of conduct) is irrelevant," he said.
The reason Iuventa was seized is because "there is a serious dangerous the criminal activity (abetting illegal immigration) will be committed again," he said.
Jugend Rettet tweeted earlier on Wednesday that it had received no official information that it was under investigation but said some of its crew had been questioned by Italian officials as a matter of routine.
In a Facebook post on Tuesday, Jugend Rettet said it wanted to continue negotiations with Rome over the code of conduct which it and other NGOs have said will hamper their life-saving operations in the Mediterranean.
The 11 point-code requires armed police on board NGO vessels, bans making phonecalls or firing flares, forbids the ships from sailing into Libyan waters unless lives are at risk, and bans the transfer of migrants from one vessel to another.
The new rules, which have been given a green light by the European Union, have also drawn criticism from rights groups including Amnesty International and from the United Nations.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
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