Russia carried out a “stunning” hack of U.S. intelligence services’ most sensitive communications, Yahoo News reports. The hack is believed to have happened around 2010 and reportedly gave Russian spies in Washington, New York, and San Francisco access to the location of FBI surveillance teams as well as the actual content of FBI communications. The hack may have allowed the Russian agents to avoid FBI surveillance, communicate with U.S. sources, and gather intelligence on their FBI pursuers, according to the report. “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 Russian spies, one source told Yahoo. The Russians reportedly compromised the encrypted radio systems used by the FBI’s mobile surveillance teams as well as the backup communications systems. “This was something we took extremely seriously,” one former senior counterintelligence official is reported to have said. The intercepts were said to be monitored by teams at the Russian diplomatic compounds that President Obama ordered seized shortly before he left office.