Russian state transportation authorities said Wednesday that the captain of one of two oil tankers involved in a major spill in the Black Sea last year did not hold valid certification at the time of the incident.

The revelation came during a court hearing in which the regional branch of the state environmental watchdog Rosprirodnadzor is suing Volgatransneft, the company that owns the tankers. The Dec. 15 spill contaminated beaches across southern Russia and sparked multiple lawsuits from local governments and federal agencies.

According to the Federal Transportation Oversight Agency (Rostransnadzor), the captain of the Volgoneft-239 tanker — whose name was not disclosed — was operating the vessel with an expired diploma. Officials said the certification had lapsed “several days” before the storm that caused the disaster.

State media did not name the captain, but previous reports identified the ship’s helmsman as 26-year-old Anastasia Yudenko, a graduate of the Saratov College of Water Transportation, Construction and Service. She reportedly trained as a navigator and assistant mechanic for small vessels.