American officials discovered that the Russians had dramatically improved their ability to decrypt certain types of secure communications and had successfully tracked devices used by elite FBI surveillance teams. Officials also feared that the Russians may have devised other ways to monitor U.S. intelligence communications, including hacking into computers not connected to the internet. Senior FBI and CIA officials briefed congressional leaders on these issues as part of a wide-ranging examination on Capitol Hill of U.S. counterintelligence vulnerabilities.

These compromises, the full gravity of which became clear to U.S. officials in 2012, gave Russian spies in American cities including Washington, New York and San Francisco key insights into the location of undercover FBI surveillance teams, and likely the actual substance of FBI communications, according to former officials. They provided the Russians opportunities to potentially shake off FBI surveillance and communicate with sensitive human sources, check on remote recording devices and even gather intelligence on their FBI pursuers, the former officials said.

“When we found out about this, the light bulb went on — that this could be why we haven’t seen [certain types of] activity” from known Russian spies in the United States, said a former senior intelligence official.