Jeff Bezos’s Blue Origin is challenging a U.S. government contract with Elon Musk’s SpaceX to develop technology to land people on the moon again.

In a complaint filed Friday in the Court of Federal Claims in Washington, Blue Origin said the National Aeronautics and Space Administration’s evaluation process was “unlawful and improper.” The complaint was filed under seal but tandem documents seeking a protective order gave an indication of its contents.

NASA selected SpaceX’s human lander bid on April 16 leading Blue Origin and a bidding partner, Dynetics Inc., to file protests with the Government Accountability Office less than two weeks later alleging that the award process was flawed. The GAO denied that protest on July 30 saying that NASA’s SpaceX selection had been made properly. Blue Origin, which has offered to contribute more than $2 billion of work on the project at no cost to the government, had vowed to continue pursuing the matter.