French prosecutors said Friday, January 23, they opened an investigation into an oil tanker intercepted in the Mediterranean Sea by the French navy and suspected of shipping Russian petroleum in violation of sanctions against Moscow.

The Grinch, an oil tanker that came from Murmansk in northwestern Russia and is suspected of being part of the sanctioned Russian shadow fleet, was redirected to an anchorage following Thursday’s operation by the French navy. Prosecutors in the southern French city of Marseille said Friday they were investigating the ship’s alleged failure to confirm its nationality.

Russia is believed to be using a shadow fleet of over 400 ships to evade sanctions over its war on Ukraine. France and other countries have vowed to crack down. The fleet is a compilation of aging vessels and tankers owned by non-transparent 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 Emmanuel Macron also linked to the shadow fleet. Putin denounced that interception as an act of piracy. The tanker’s captain will go on trial in February over the crew’s alleged refusal to cooperate, according to French judicial authorities.