WASHINGTON – Secret Defense Department studies cast doubt on whether a multibillion-dollar missile defense system planned for Europe can ever protect the U.S. from Iranian missiles as intended, congressional investigators say.

Military officials say they believe they can overcome the problems and are moving forward with plans. But proposed fixes could prove difficult. One possibility has been ruled out as technically unfeasible. A second, relocating missile interceptors planned for Poland and possibly Romania to ships on the North Sea, could be diplomatically troublesome.

The studies are the latest to highlight serious problems for a plan that has been criticized on several fronts.