Two Greek shipping companies will pay a multi-million dollar fine and two crew members face possible prison time after discharging oil into two Texas ports, then lying to the US Coast Guard about it.

Vardinoyiannis Group company Avin International, shipowner Nicos IV Special Maritime Enterprises, master Rafail-Thomas Tsoumakos and chief officer Alexios Thomopoulos all pleaded guilty to charges stemming from three illegal releases of oil last summer in Beaumont, Texas federal court Tuesday, according to the US Justice Department.

Avin and Nivos IV Special Maritime will pay a $4m fine and spend four years on probation. Tsoumakos and Thomopoulos could spend up to five years behind bars. A sentencing date for the two men has not been set.

“The international ports of Houston and Port Arthur are no one’s dumping ground,” Assistant Attorney General Clark said in a statement.

“Vessel operators coming to the United States must not foul American waterways. Those who knowingly discharge their waste and lie to the Coast Guard to dodge their legal responsibilities under federal law are on notice that our investigators and prosecutors stand ready to hold them accountable.”

Dimitri Georgantas, attorney for Avin, did not immediately return requests for comment.

According to the Justice Department, oil had gotten into the segregated ballast system on the 45,000-dwt product tanker Nicos IV (built 2002) and the ship discharged it twice in the Port of Houston on 6 and 7 July. The discharge was not recorded.

The ballast tanks were opened again en route to Port Arthur and, once in the port, oil began bubbling up next to the vessel. The Coast Guard investigated and both Tsoumakos and Thomopoulos told officials they had not been aware of the oil.

The two companies pleaded guilty toone count of obstruction of an agency proceeding, one count of failure to report discharge of oil and three counts of negligent discharge of oil. The mariners pleaded guilty to making a material false statement.