A U.S. federal grand jury in Charleston indicted a Liberia shipping company, a corporate vessel owner and three engineering officers on Friday in connection with a longstanding environmental conspiracy investigation dating back to last August.

The charges relate to an August, 2015 case when a Liberian-flagged chemical tanker, the T/V Green Sky, entered the Port of North Charleston on the eastern coast of South Carolina.

At the time, members of the vessel allegedly told U.S. Coast Guard officials that they had been ordered to bypass the ship’s oil water separator on several occasions.

The API separator is a device that is designated to break apart oil and other suspended solids from wastewater to help remove detrimental chemical compounds in the case of an oil spill.

According to court filings, Aegean Shipping Management of Liberia, the ship’s management company and the engineering officers aboard the vessel, violated international laws by illegally discharging oil and space bilge water from the tanker.

As parties to the MARPOL treaty, vessels in the U.S. and Liberia are required to record all discharges in their ship’s oil book. On Friday, the shipping company and the officers were charged with obstruction, conspiracy and falsifying records by providing inaccurate information in the oil book.

In February, Genaro Anciano, the former captain of the tanker pleaded guilty to obstruction of justice charges for providing misleading information to officials in the Coast Guard’s investigation. Separately, it is also a federal crime for international vessels to enter a U.S. port with false records in an oil book.

The three engineering officers, Panagiotis Koutoukakis, Herbert Julian and Nikolaos Bounovas, were charged with failure to maintain an accurate oil record book, falsification of federal records and aiding and abetting a federal crime.

The defendants are scheduled to be arraigned in Charleston on July 26. The case was investigated by the Coast Guard’s Investigative Service.