(Bloomberg) — More than a month since oil started washing up on some of Brazil’s most touristic beaches, dotting sand with black patches, killing sea turtles and scaring off fishermen, the origin of the crude is still a mystery.

“We don’t know the oil’s origin, where it came from or how it got here,” Energy Minister Bento Albuquerque said at an offshore exploration auction in Rio de Janeiro on Thursday.

The crude probably leaked from a ship in the ocean, he ventured, adding that it has characteristics similar to Venezuelan heavy crude — which doesn’t mean it comes from there. Venezuela’s state oil company categorically denied having anything to do with the slick, saying there were no reports of incidents at its facilities or from clients, nor evidence of leaks that could have led to damages in Brazil.

The massive spill has already spread along the coasts of all nine states in Brazil’s northeast. Over a dozen sea turtles have been found dead, covered in crude, local newspaper O Estado de S. Paulo reported. Some 800 baby turtles that hatched were kept from going into the sea, the newspaper said, citing Projeto Tamar, one of Brazil’s best-known wildlife conservation projects.

The nation’s environmental agency said the oil found on the beaches was not produced by Brazil, and that the country’s Navy and federal police are investigating the spill.

On Wednesday, Environment Minister Ricardo Salles said the oil likely originated from Venezuela, citing a report from state-controlled oil company Petroleo Brasileiro SA about the characteristics of the crude.

Petrobras Chief Executive Officer Roberto Castello Branco said Tuesday that the spill could have come from an oil tanker that sank, an accident when loading oil from one tanker to another, or from a criminal act. President Jair Bolsonaro has said for days that the oil spill was probably criminal, without elaborating further.

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