The Justice Department has expanded a lawsuit against the real estate software company RealPage, adding six of the biggest landlords and property management companies in the country to the suit and alleging that they coordinated to keep rents artificially high.

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.

Greystar, Blackstone, Camden Property Trust and Cushman & Wakefield are among the firms added to the suit that alleges RealPage enabled landlords to fix rents.

Millions of rents across the country may now be set using one company’s algorithmic software, according to a federal lawsuit and a Washington Post analysis.