WASHINGTON, May 14 (Reuters) - Seven & I Holdings Co Ltd's announcement it had pressed forward in its deal to buy 3,900 Speedway gas and convenience stores from Marathon Petroleum Corp is potentially illegal and may raise significant competitive concerns in hundreds of local gasoline and diesel fuel markets across the United States, two U.S. Federal Trade Commission officials said on Friday.

"The parties have closed their transaction at their own risk. The Commission will continue to investigate to determine an appropriate path forward to address the anticompetitive harm and will also continue to work with State Attorneys General," said Rebecca Kelly Slaughter, the FTC's acting chairwoman, and Rohit Chopra, an FTC commissioner, in a statement. (Reporting by Makini Brice Editing by Chris Reese)