A US grand jury has indicted Germany's MST Mineralien Schiffahrt Spedition und Transport on charges that it falsified records to cover up oily discharges from a ship.

The charges against the shipmanager and the registered owner of the 27,700-dwt slurry carrier Marguerita (built 2016) were lodged in a federal court in Portland, Maine.

They allege violations of the Act to Prevent Pollution from Ships and falsification of records with the attempt to impede US Coast Guard examinations.

The indictment comes slightly more than a year after the company pleaded guilty to similar charges.

The company could not be immediately reached for comment on the charges, and a lawyer declined to discuss the case.

But court papers obtained by TradeWinds show that the company voluntarily disclosed to US authorities that a chief engineer had made an unknown and unwanted discharge of bilge water. MST took remedial measures and launched an investigation.

The chief engineer was fired, a source with knowledge of the case said.

Nonetheless, the ship was boarded last month by the USCG and detained.

The Justice Department announcement says the indictment stems from alleged falsification of records between 2016 and 2017 on the Liberian-flag Marguerita.

"On at least eight occasions between September 2016 and June 2016, the MV Marguerita entered United States waters and ports with a false and misleading oil record book available for inspection by the US Coast Guard," the agency said. "The oil record book failed to accurately record transfers and discharges of oily wastewater on the vessel."