Oil giant BP Plc's shares sank further on Monday as the oil spill in the Gulf of Mexico continued to grow and the U.S. Justice Department said it was taking part in an investigation of the disaster.

British markets were closed on Monday but BP's Frankfurt-listed shares opened 8 percent lower and were down 7 percent at midday, against a 0.3 percent fall in the STOXX Europe 600 oil and gas index <.SXEP>.

In New York, American Depositary Receipts of BP's shares fell 7.8 percent to $48.10, bringing their drop since the close of trading on April 23, to nearly 20 percent.

U.S. Attorney General Eric Holder's comments that the Justice Department was involved in the probe raised the specter of criminal liability for BP in the oil spill spreading toward the Gulf Coast, after an explosion at a rig drilling its oil well on April 20.

BP's losses are on top of a drop of around 13 percent since the accident, equating to a drop of around $20 billion in BP's market capitalization.

The problem with BP is, as long as the leak continues, nobody knows how much it'll cost to clean up the mess, so the stock could continue to sink for a while, one Paris-based trader said.

President Barack Obama visited Louisiana over the weekend and laid the cost of the clean up firmly on BP.

BP has not put an estimate on the likely costs of the spill, which follows the explosion and sinking of a drilling rig operated by Swiss-based driller Transocean.

The company said last week it and partners in the well, including Anadarko Petroleum , were paying $6 million/day in clean-up efforts, but admitted costs would rise sharply when the oil hit land, as would damages claims.

Shares in other companies linked to the spill also weakened. Transocean Ltd , the owner of the drilling rig, saw its shares fell 6 percent to 74.75 Swiss Francs in Geneva, Halliburton shares slipped 2.4 percent to $29.91 in New York, and Cameron International fell 2.9 percent to $38.33.

(Reporting by Matt Daily and Tom Bergin in London, editing Tim Dobbyn)