Federal prosecutors allege that the landlords have used RealPage pricing software to collude and artificially raise rents. The legal action is the latest development stemming from a 2022 ProPublica investigation.

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 so prices — and profits — would soar. Officials conducted a two-year investigation into the scheme following a 2022 ProPublica story that showed how RealPage was helping landlords set rents across the country in a way that legal experts said could result in cartel-like behavior.

Together, the six landlords manage more than 1.3 million apartments in 43 states and the District of Columbia. Prosecutors have already negotiated a settlement with one of them.