A Chinese commercial vessel that has been surrounded by European warships in international waters for a week is central to an investigation of suspected sabotage that threatens to test the limits of maritime law—and heighten tensions between Beijing and European capitals.

Investigators suspect that the crew of the Yi Peng 3 bulk carrier—225 meters long, 32 meters wide and loaded with Russian fertilizer—deliberately severed two critical data cables last week as its anchor was dragged along the Baltic seabed for over 100 miles.