The inquiry could lead to a variety of possible violations, including manslaughter charges in the deaths of 11 workers killed when the Deepwater Horizon rig exploded April 20, 2010, The Washington Post reported Wednesday.
Charges aren't imminent and law enforcement and other sources told the Post it was unclear whether they would be filed against the companies -- BP or Transocean, which leased the rig to BP -- or against individuals.
"I'd be surprised if they don't prosecute BP and Transocean," said David Uhlmann, a University of Michigan law professor and the Justice Department's former top environmental crimes prosecutor. "Whether individuals can be charged is a much more difficult question."
Officials from BP, Transocean and the Justice Department declined to comment, the Post said.
Separately, a civil inquiry led to the Justice Department suing BP and eight other companies in December. The lawsuit accuses BP and other firms of violating federal regulations and failing to prevent the explosion and seeks civil penalties under federal environmental statutes.
The environmental disaster began with the oil rig explosion April 20 and the spill continued until July 15. The spill released an estimated 206 million gallons of crude oil and is considered the largest accidental marine oil spill in petroleum industry history.