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 Delta Tankers Ltd last week.
Delta Tankers told AFP they did not accept blame for the spill and the tanker's voyage ended "uneventfully".
Marine life and popular beaches in a number of north-eastern Brazilian states have been affected by the spill.
The tanker alleged to have caused the spill, the Bouboulina, was carrying heavy crude oil from Venezuela to Malaysia in July.
But Delta Tankers said the Bouboulina "completed her voyage uneventfully, without having experienced any fuel shortage".
"There is no proof of the vessel having stopped, conducted any kind of STS (Ship to Ship) operation, leaked, slowed down or veered off course on its passage from Venezuela to Melaka, Malaysia," the company said in a statement.
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".