President Barack Obama has invoked executive privilege and the attorney general has been found to be in contempt of Congress for refusing to turn over documents that might explain what led the Justice Department to reverse course after initially denying that federal agents had used a controversial tactic called gun-walking in the failed law enforcement operation. The tactic resulted in hundreds of illegally acquired weapons purchased at Arizona gun shops winding up in Mexico, where many of them were recovered from crime scenes. Two guns in Operation Fast and Furious were found on the U.S. side of the border at the scene of a shooting in which U.S. border agent Brian Terry was killed. In a Feb.