British oil company BP could face a criminal investigation in the US over a massive oil spill in Alaska in March.
The spill covered two acres of the snow-covered tundra
The Financial Times newspaper has obtained internal emails which show it is under investigation by a grand jury.
The emails reveal BP has been ordered to hand over a number of documents and other data relating to the leak thought to be caused by a corroded pipe.
A spokesman for the company said BP would provide information showing it had acted properly.
The spill of some 267,000 gallons (1m litres) discovered at Prudhoe Bay field, is the largest ever on Alaska's North Slope region.
BP Alaska, which operates the pipeline, has denied claims it failed to maintain it properly.
Such an investigation by a grand jury could lead to criminal charges and ultimately fines and prison sentences.
The spill was detected on 2 March and plugged. Local environmentalists have described it as "a catastrophe".
BP is already facing a probe into a blast at a Texas refinery in March 2005 in which 15 people died and 170 were injured.
BP said "management failures" and "employee mistakes" were responsible.
In 1989, the Exxon Valdez shipping disaster spilled 11m gallons (42m litres) of oil onto the Alaskan coast.