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.

The Justice Department is adding six major apartment landlords to its lawsuit against real-estate software company RealPage, expanding the scope of its allegations that the rent-setting algorithm enabled illegal price fixing.