Zillow allegedly paid Redfin $100 million to stop competing against it in online apartment rental listings, the U.S. Federal Trade Commission said in a lawsuit Tuesday against the online real estate platforms.

The agency said the alleged deal reduces competition in an already concentrated market and is likely to drive up the cost of advertising vacancies in multifamily rental buildings.

“Paying off a competitor to stop competing against you is a violation of federal antitrust laws,” said Daniel Guarnera, who leads the FTC’s bureau of competition.