Spanish authorities have arrested 15 crew after 4 tonnes of cocaine was discovered on their platform supply vessel (PSV).
Domestic media said the 1,200-dwt Karar (built 1991) was intercepted 320 km from the coast of Galicia.
The vessel was brought into Vigo in Spain this week.
The crew consisted of Bangladeshis and Nepalese. A Spanish national was also on board.
Another 13 arrests were made in the country as the drug smuggling ring was rounded up.
The Togo-flagged ship has not shown up on radar since last July when it was in Cristobal, Panama.
The El Correo Gallego newspaper said the raid was a joint operation between Spain's National Police and the Customs Surveillance Service.
The Faro de Vigo website reported it also involved the Spanish Navy, the US Drug Enforcement Agency, Colombian police and the UK National Crime Agency.
Six-month operation
A dozen officials from the tax agency and the Special Operations Group of the National Police boarded the Karar.
The smugglers' plan was to send two boats out to the ship and unload the drugs somewhere in the the Arousa estuary, the reports said.
The investigation began six months ago, focusing on a group made up of experienced sailors and well-known traffickers.
The gang was watched in Spain. They were involved in the construction of boats and small fishing vessels or the collection of fuel and food.
Constant movement of speedboats to hide them around the coast alerted officials that the final stages of a big operation was taking place.
The vessel had allegedly loaded the drugs at the start of April and had been in transit from South America for 20 days.
Police said that the operation remains open and no further arrests are ruled out.
The vessel has no updated ownership information since July 2019 and was listed as laid up at that time. The last owner listed is Marcomer SA, which could not be contacted.
Karar has no port state detentions listed among five checks made in Cartagena, Colombia, during 2018 and 2019.
