A Greek-flagged ship carrying Venezuelan crude was the source of an oil spill which has tarred thousands of kilometers of coastline over the past two months, Brazilian investigators have announced.

Police said the tanker appears to have spilled the crude about 700km (420 miles) off Brazil’s coast between 28 and 29 July, bound for Singapore with oil loaded at Venezuela’s San José terminal.

Brazil’s solicitor general said the country would seek damages in the case, which has stained tropical beaches along 2,500km of coastline with a thick sludge, hurting tourism and fishing communities in the poorer north-east region.

The attorney general’s office said the damage the spill had caused on Brazil’s coasts was “immeasurable”.

“There is strong evidence that the company, the captain and the vessel’s crew failed to communicate with authorities about the oil spill/release of the crude oil in the Atlantic Ocean,” prosecutors said in a statement.

Federal police also carried out search warrants at offices in Rio de Janeiro linked to two companies with commercial relationships with the ship’s operator.

Brazilian authorities said they had also requested cooperation from international agencies, including Interpol, to further investigate the ship, its crew and the company.

Federal prosecutors said Brazil’s navy also had information regarding a prior detention of the vessel in the United States for four days due to “incorrect operating procedures related to the separation of oil and water for release in the sea”.

It was unclear when the US detention occurred.

The police said oceanographic and geolocation data showed that the Greek ship was the only one navigating near the origin of the spill between 28 and 29 July, after docking in Venezuela around 15 July.

From late August to the end of October, the oil had washed ashore in nine states and 94 cities, according to federal police, killing scores of animals and closing hundreds of beaches.

Brazil has so far collected some 2,000 tonnes of sludge from its beaches in continuing cleanup efforts, while working to rehabilitate birds and sea turtles coated in the thick crude.

The slow and patchwork cleanup efforts, along with weeks of confusion about the cause of the spill have spurred criticism of the Brazilian government’s response. Officials have said Brazil is following standard protocols since the start of the disaster.

Because the heavy crude does not float on the ocean surface like most oil slicks, officials said traditional methods of tracking it and keeping it off the shore have been ineffective.