The FTC (U.S. consumer protection agency) is preparing possible action against Booking Holdings over online advertisements that appear to send consumers to hotel websites but instead redirect them to Guest Reservations, a third-party booking site that often charges additional fees, two sources familiar with the matter told Reuters. The potential case could involve more than $500 million in penalties. The agency is examining whether Guest Reservations websites impersonate hotels or violate other rules on fees and related practices. Booking disclosed in August that the FTC was considering suing Priceline and an unnamed affiliate over disclosures, fees, customer support and billing, and said it was negotiating a resolution. More than 1,000 complaints have been filed against Guest Reservations with the Better Business Bureau, while an ongoing San Francisco lawsuit says Booking provides the company with hotel inventory through Priceline Partner Solutions. The investigation comes as the FTC expands consumer-protection enforcement against misleading online advertising, including ads that make third-party sites appear hotel-operated.