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.

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.

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 lawsuit arrives as U.S. renters continue to struggle under a merciless housing market, with incomes failing to keep up with rent increases.

The U.S. Department of Justice is suing several major landlords for allegedly colluding to keep rents high. The lawsuit accuses them of using an algorithm to set rents and sharing sensitive pricing information to avoid competition.