The Financial Times in London said it had obtained an e-mail dated April 26 from the president of BP Alaska, Steve Marshall, which told employees that the company had received a subpoena from a federal grand jury in Alaska.
The grand jury had asked for a "variety of documents and data from BP Alaska concerning the transit line and certain other operational areas", the newspaper said, adding that the e-mail requested staff to treat it as confidential.
Up to one million litres of oil leaked from a damaged pipeline in the United States' largest oil field in Prudhoe Bay, about 1,040 kilometres north of Alaska's biggest city, Anchorage, on March 2.
The leak, which is the worst spill in the region's history, is thought to have been caused by metal corrosion. BP has denied allegations that it failed to maintain the pipeline, according to the FT.
Mr Marshall reportedly told staff in the correspondence that the company was fully committed to co-operating with the request and urged them to do the same.
The newspaper quoted BP spokesman Daren Beaudo as confirming the e-mail, adding: "I believe that the information we provide will show that the actions of BP Alaska were, at all times, proper."
If the investigation goes against BP, it could result in prison terms, significant fines plus increased controls on its operations, at a time when its US operation was under scrutiny.
In May last year, the company accepted responsibility for a fatal explosion in March 2005 at its Texas City refinery in the United States, in which 15 people died and more than 170 were injured.