WASHINGTON (AP) — The U.S. government’s efforts to determine which highly classified materials leaker Edward Snowden took from the National Security Agency have been frustrated by Snowden’s sophisticated efforts to cover his digital trail by deleting or bypassing electronic logs, government officials told The Associated Press. Such logs would have showed what information Snowden viewed or downloaded.

The government’s forensic investigation is wrestling with Snowden’s apparent ability to defeat safeguards established to monitor and deter people looking at information without proper permission, said the officials, who spoke on condition of anonymity because they weren’t authorized to discuss the sensitive developments publicly.