The U.S. Justice Department is suing six major U.S. landlords for allegedly participating in algorithmic pricing schemes harming millions of renters.

Federal prosecutors sued the real estate software company RealPage in August, now they're suing some of the company's biggest customers.

Several large landlords allegedly colluded in an effort to keep apartment rents high, including by using an algorithm that influences prices, according to new legal filings by the Colorado attorney general,

The real estate companies kept rents high with RealPage's software and by sharing information, the antitrust complaint says.

The complaint alleged the six companies set their rents using each other's information by either speaking with competitor's senior managers, calling around, and participating in user groups to discuss how to modify pricing methods.

The U.S. Department of Justice and 10 state attorneys general have ramped up their antitrust lawsuit against RealPage, a property management software compa