New Delhi, Dec. 18 -- France's counterespionage agency is investigating a suspected cyberattack plot targeting an international passenger ferry, authorities said Wednesday.

A Latvian crew member is in custody facing charges of having acted for an unidentified foreign power, French officials said. But Interior Minister Laurent Nunez appeared to hint that Russia is suspected, saying: "At the moment, foreign interference very often comes from same country."

France and other European allies of Ukraine allege that Russia is waging "hybrid warfare" against them, using sabotage, assassinations, cyberattacks, disinformation and other hostile acts that are often hard to quickly trace back to Moscow.

Intelligence shared by Italian authorities ti...