The oil giant BP is under criminal investigation in the US for a big oil spill in Alaska in March that has raised fresh questions about the company's safety record.
BP confirmed that BP Alaska's president, Steve Marshall, sent an email to employees on May 30 reminding them to cooperate with investigators after the company received a grand jury subpoena on April 26.
"The incident happened in March, and it has been clear that a criminal investigation has been under way for some time," said a BP spokesman, David Nicholas.
He said Mr Marshall had sent an email, first reported in the Financial Times, reminding staff to cooperate with investigators. He noted that Mr Marshall had said he believed any information provided would show that the "actions of BP Alaska were, at all times, proper".
BP received a subpoena on April 26 from a federal grand jury in Alaska, ordering the company to hand over documents in connection with the oil leak. Grand juries decide whether there are grounds to indict a company or individuals.
BP's safety record is already being criticised after an explosion at the company's Texas City, Texas, refinery that killed 15 workers. The March 2005 explosion led to a $21.4m (£11.5m) fine, the largest ever imposed by US safety regulators.
John Browne, BP's chief executive, was paid a lower performance bonus in 2005 compared with a year earlier, partly because BP's safety performance was "impaired".
Lord Browne received £1.7m last year, down from £2.2m in 2004.
Environmentalists say the oil spill in Alaska raises serious questions about BP's environmental standards. Friends of the Earth has also raised concerns about the risk of leaks on the Baku Ceyhan pipeline, which goes through Azerbaijan, Georgia and Turkey.
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