The Metro’s six-station Silver Line extension project has been delayed an additional two months, according to a progress report from the Metropolitan Washington Airport Authority.

Hensel Phelps, the company in charge of building the project’s rail yard, fell 67 more days behind schedule in March and April, putting the company more than a year and a half behind, while Capital Rail Constructors, the contractor building the tracks and stations, is eight months behind schedule, WAMU reported Friday. The extension is the first to be built by private companies rather than the Washington Metropolitan Area Transit Authority.

Will Thompson, the vice president and district manager for Hensel Phelps, told WAMU that “testing criteria” are causing delays in the project.

The completion of the Silver Line extension will expand Metro rail service to Washington Dulles International Airport and Ashburn. Contractors expect the project to be finished by July 2020, but MWAA has not approved that end date, WAMU reported.

The Washington Post reported several problems with the construction project in April, including cracks in the concrete pedestals at Dulles Airport and defective rail ties. Keith Couch, Capital Rail Constructors’ project director, told WAMU that the company will begin repairs to the pedestals and rail ties in the next two weeks.

MWAA spokeswoman Maria McAllister told WAMU that Hensel Phelps originally projected the extension would be complete on Dec. 18, 2018. She said MWAA is negotiating with the company for “the costs of the delays.”

Capital Rail Constructors may face a similar penalty if it fails to substantially complete its part of the project by Aug. 7 of this year, WAMU reported.