Under international maritime law, NATO ships can't force the Yi Peng 3 to sail into one of their ports. Swedish and German authorities are negotiating with the ship's owner to obtain access to the vessel and question its crew.
German police also dispatched the Bamberg, a patrol vessel, to investigate one of the incidents with underwater drones. Swedish and Danish ships have also examined the sites on the seabed.
European authorities must tread carefully because of their commitment to the freedom of navigation and upholding international law that underpins global trade, according to several European politicians, as well as security and law-enforcement officials familiar with the probe.