7-Eleven Inc.’s purchase of the Speedway retail chain violates antitrust laws, the head of the U.S. Federal Trade Commission said, casting doubt over the future of the $21 billion deal that closed Friday.

FTC Acting Chairwoman Rebecca Kelly Slaughter and her fellow Democratic commissioner said the agency would continue to investigate the acquisition even after 7-Eleven announced it had completed the deal.

“We have reason to believe that this transaction is illegal,” Slaughter and Commissioner Rohit Chopra said in a statement. The “decision to close under these circumstances is highly unusual, and we are extremely troubled by it.”

7-Eleven’s parent, Tokyo-based Seven & i Holdings Co., agreed in August to buy 3,900 Speedway outlets from Marathon Petroleum Corp. to clinch a dominant position of almost 14,000 stores in the U.S. and Canada. The transaction gives 7-Eleven a presence in 47 out of the top 50 metropolitan markets.

The transaction followed months of pressure on Marathon from investors including Elliott Management Corp. and D.E. Shaw & Co., for sweeping changes to improve its performance. Elliott had pushed for Marathon to break itself up into three separate businesses: refining, retail and pipelines.