The Justice Department today took a major step in its criminal investigation of the Deepwater Horizon oil spill by filing charges against a former BP PLC engineer.

The two counts of obstruction of justice filed against Kurt Mix in the U.S. District Court for the Eastern District of Louisiana are the first criminal charges filed in the massive investigation of the events surrounding the April 20, 2010, explosion and subsequent oil spill at the Macondo well in the Gulf of Mexico.

The government claims Mix, who worked on various efforts to stop the leak, deleted more than 200 text messages on his cellphone.

The messages, featuring what DOJ calls "sensitive internal BP information collected in real time," include exchanges between Mix and a BP supervisor during the failed attempt -- known as the "top kill" -- to pump heavy mud into the wellhead in an effort to stop the leak.

Mix deleted messages that showed the operation was not working, which is the opposite of what BP said in public at the time, the government alleges. The messages also indicated that the flow of oil was much higher than what BP reported, DOJ says.

Prosecutors also claim Mix deleted another string of exchanges with a BP contractor that discuss the flow rate from the well immediately after the explosion.

According to a DOJ statement, Mix deleted the messages in August and October 2010 after being told not to by BP lawyers. He also knew a criminal investigation was under way and had communicated with a criminal defense lawyer.

The fact that both charges relate to discussion of the flow rate is significant because it is a "central question" in the government's parallel civil investigation, noted David Uhlmann, who oversaw prosecution of environmental crimes at DOJ before becoming a professor at the University of Michigan School of Law.

BP has negotiated a settlement with private plaintiffs in the civil litigation, but it has not reached a similar deal with the government (Greenwire, March 5).

Establishing the flow rate will have a sizable impact on the civil penalties BP would have to pay. Under the Clean Water Act, BP faces penalties of at least $1,100 per barrel spilled. That amount could rise to $4,300 a barrel if the government could prove the company was grossly negligent.

According to an affidavit attached to today's complaint, the deleted messages show that Mix indicated the flow rate during the top kill operation was more than 15,000 barrels of oil a day, rather than BP's public estimate at the time of 5,000 barrels a day.

Jane Barrett, director of the environmental law clinic at the University of Maryland Carey School of Law, said the focus on the top kill operation "raises the question of whether BP misled the government on key or critical facts during that phase of the effort to cap the well."

As for the criminal investigation, Barrett -- a former federal prosecutor -- said other BP employees could face false statement charges if what Mix was saying internally is similar to what BP told the government at the time.

A key issue will be whether Mix is able to provide the government with evidence implicating others as part of a plea bargain, she added.

Uhlmann said he expects more criminal charges to follow "in the near future."

In a statement, Attorney General Eric Holder said DOJ's Deepwater Horizon task force "is continuing its investigation into the explosion and will hold accountable those who violated the law in connection with the largest environmental disaster in U.S. history."

BP said in its own statement that it had "clear policies requiring preservation of evidence in this case and has undertaken substantial and ongoing efforts to preserve evidence."

The company declined to comment on the specifics of the charges against Mix.

Click here to read the criminal complaint.

Click here to read the affidavit that accompanies the complaint.