Attorneys generals from 10 states also joined the suit, claiming the companies hurt "millions of American renters."

An amended complaint names six landlords as defendants in ongoing litigation that North Carolina and several other states, along with the federal government, began in August.

The U.S. Justice Department is suing several large landlords for allegedly coordinating to keep Americans’ rents high

The Justice Department is adding six major apartment landlords to its lawsuit against real-estate software company RealPage, expanding the scope of its allegations that the rent-setting algorithm enabled illegal price fixing.

The U.S. Department of Justice filed an amended complaint Tuesday alleging six of the nation’s largest landlords — including Dallas-based Willow Bridge Property Co. — participated in an illegal scheme allowing landlords to increase rental prices and decrease competition.