The Department of Justice on Tuesday sued six of the nation’s largest landlords, accusing them of using a pricing algorithm to improperly work together to raise rents across the country. The lawsuit expands an antitrust complaint the department filed in August that accused property management software-maker RealPage of engaging in illegal price-fixing to reduce competition among landlords

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