The Justice Department has threatened to file criminal charges against a former top National Archives official if he testifies as a defense witness in a high-profile national security case. J. William Leonard, who resigned last January as the U.S. government’s chief expert on classified information, after a bitter clash with Vice President Dick Cheney’s office, has been cooperating with defense lawyers in a case charging two former officials of the American Israel Public Affairs Committee (AIPAC) with improperly disclosing national security secrets, according to recently filed court papers.