The NSA has been recording and gathering private phone calls for years, but it used to be difficult for the agency to identify unknown speakers. In the past, signals intelligence (SIGINT) transcribers worked on the same targets for years before they became familiar enough with a speaker’s unique voice to be able to verify their identity.

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Now, the NSA is using more advanced computational systems developed by the Massachusetts Institute of Technology (MIT) in order to catch spies and terrorists, according to a declassified NSA document obtained by media outlet the Intercept.

The document describes how the NSA used the technology during Operation Iraqi Freedom to match an audio recording to former leader Saddam Hussein’s “voiceprint.” The NSA also used the technology to compare the voice of a captured suspect with previous audio recordings from terrorist Abu Hakim to confirm that the suspect was not a match.

In order to test their technology, analysts at the NSA compared old intercepts and audio files relating to Ron Pelton, a former NSA analyst who sold details about several secret US programs to the Soviet Union. At the time, the agency failed to identify Pelton through human voice identification. However, in 2006, the agency was able to automatically match Pelton’s voice using the technology.

“Had such technologies been available twenty years ago, early detection and apprehension could have been possible, reducing the considerable damage Pelton did to national security,” the document states.