Brazilian officials suspect a Greek-flagged tanker was the source of an oil spill that has stained about 2,500km (1,553 miles) of Brazil’s coastline.
Federal police in the city of Rio de Janeiro raided the offices of a Greek company linked to the vessel.
Officials believe the tanker was carrying heavy crude oil from Venezuela to South Africa in July.
Marine life and popular beaches in a number of north-eastern Brazilian states have been affected by the spill.
About 2,000 tonnes of thick sludge have been collected, but a huge clean-up effort is continuing.
Concerns are growing that the oil spill could reach the Abrolhos islands, an important marine sanctuary.
The spill was first detected on 2 September, and analysis later showed the oil found was of a type not produced in Brazil, officials say.
It was “very likely from Venezuela,” the Brazilian environment ministry said.
Venezuela has denied any responsibility for the spill.
The Brazilian authorities have investigated a number of ships navigating the area at the time before focusing on the Greek-flagged tanker. They describe the vessel as the “prime suspect”.
The company whose offices were raided on Friday has so far made no public comment on the issue.