A senior Metro official two weeks ago accused Dulles Rail project managers of failing to resolve a series of technical problems that are potentially serious enough to prevent the transit agency from operating the proposed line, letters between the two agencies show.

The most recent letter, a Jan. 16 memo from John Thomas, director of Metro’s office of major capital projects, highlights a behind-the-scenes rift over dozens of complex engineering issues and illustrates one of the reasons why the Federal Transit Administration says it won’t fund a critical $900 million for the project’s first phase.