The judge did not limit his findings to Keystone, so the ruling is being interpreted for now to apply to any project using Nationwide Permit 12. The Montana case was cited last week in a lawsuit over a 430-mile (692-kilometer) natural gas pipeline in central Texas. Plaintiffs including the city of Austin said last week's ruling invalidated the Army Corps' work on the Permian Highway Pipeline.

Because there are no streams or other water bodies near the northern Montana border crossing where Keystone XL is currently under construction, that work was not immediately halted. Yet it's an obstacle to future work given the many water crossings along the line's path.

Army Corps regulatory program Chief Jennifer Moyer said in an April 17 email obtained by AP that "out of an abundance of caution" she was ordering agency personnel across the nation to stop verifying companies as compliant with the blanket permit.

Questions about the directive were referred to U.S. Department of Justice spokesman Wyn Hornbuckle, who declined comment because the case is still in litigation.