Federal regulators on Friday said a group of unknown individuals used trading accounts at foreign brokerage firms to conceal their identities and illegally profit from advance knowledge of the proposed $32 billion buyout of electric utility TXU Corp.

The Securities and Exchange Commission filed a civil lawsuit in federal court in Chicago alleging illegal insider trading in connection with what would be the largest private buyout in U.S. history. “Highly profitable and suspicious” purchases of options on TXU stock on the Chicago Board Options Exchange occurred last month prior to the Feb. 26 announcement that the company had signed a deal to be acquired by private equity firms, the SEC said.