Colorado Attorney General Phil Weiser and federal regulators filed a joint lawsuit against the largest apartment owner in the country on Thursday, accusing it of intentionally misleading tenants about the cost of its rentals.

Colorado and the FTC say Greystar Real Estate and Property fraudulently advertises low rent prices to potential customers, only to add tens if not hundreds to the monthly rent cost with hidden fees.

The antitrust agency recently created a rule regarding junk fees, but its application to the multifamily sector is unclear.