U.S. District Judge John Bates ruled Friday the Justice Department must provide an index of documents in the case involving the former Republican senator from Nevada and explain why each is being withheld from Citizens for Responsibility and Ethics, a Washington advocates group that was rebuffed by several government agencies.
The group is to receive the list, called a Vaughn Index.
"Bates handed CREW a major victory," the group said in a statement posted on its website.
"A special counsel hired by the Senate Select Committee on Ethics had issued a report finding the senator's conduct appeared to violate numerous federal laws."
Ensign claimed in 2010 he was no longer the subject of a criminal investigation that began after he publicly admitted he had been having an affair with a former campaign staffer, Cynthia Hampton, later identified as the wife of his former chief of staff Doug Hampton.
The investigation included allegations he violated federal laws and Senate ethics rules in attempting to cover up the affair.
CREW sought information from the Justice Department, the FBI and the U.S. attorney's office, through the Freedom of Information Act, but was denied until the U.S. District Court of the District of Columbia granted the request Friday, the CREW website said.