PARIS (AP) — The captain of a tanker intercepted in the Mediterranean Sea by the French navy on suspicion of shipping oil in violation of sanctions against Russia was being held in custody on Sunday for questioning.

The ship’s Indian captain, 58, was handed to judicial authorities following the diversion of the oil tanker, Grinch, and its arrival at anchorage in the Gulf of Fos-sur-Mer, the Marseille prosecutor’s office said in a statement.

The investigation is being conducted by the Maritime Gendarmerie’s Investigation Unit in Toulon, jointly with the Marseille Ship Safety Centre, on charges of failure to fly a valid flag, according to the statement, which added that the crew, also of Indian nationality, was being kept on board.

“The purpose of the investigation is to verify the validity of the flag flown by the tanker and the documents required for its navigation,” the statement said.

The Grinch came from Murmansk in northwestern Russia and is suspected of being part of the sanctioned Russian “shadow fleet.” A video provided by the French military showed members of the navy boarding the ship from a helicopter earlier this week.

Russia is believed to be using a fleet of over 400 ships to evade sanctions over its war on Ukraine. France and other countries have vowed to crack down.

The fleet comprises aging vessels and tankers owned by nontransparent entities with addresses in non-sanctioning countries, and sailing under flags from such countries.

Last September, French naval forces boarded another oil tanker off the French Atlantic coast that President Emmanuel Macron also linked to the shadow fleet. Putin denounced that interception as an act of piracy.

That tanker’s captain will go on trial in February over the crew’s alleged refusal to cooperate, according to French judicial authorities.