Shares of Chesapeake were underwater after announcing that it is the subject of a U.S. government investigation over possible criminal antitrust violations related to the purchase and lease of oil and gas properties in Michigan. According to a filing with U.S. regulators on Thursday, Chesapeake received a subpoena from the antitrust division of the Justice Department's Midwest field office, requiring the company to produce documents before a grand jury in the Western District of Michigan.

In June, Reuters reported that Chesapeake plotted with its top competitor, Encana (ECA), to suppress land prices in the Collingwood shale in Northern Michigan. Chesapeake said it had held talks with Encana but that no deals were consummated between the companies and that they never bid jointly.

The Justice Department is "moving criminally," said Darren Bush, a former antitrust attorney for the Department of Justice and a professor of antitrust law at the University of Houston. "They are working their way through the grand jury process to potentially serve up indictments."

Chesapeake Energy (CHK : NYSE : US$19.65), Net Change: -0.66, % Change: -3.25%, Volume: 16,652,249