Operating in secrecy in a nondescript glass-and-concrete office, the team of prosecutors and investigators since May 2017 has unearthed secrets that have led to bombshell charges against several of President Donald Trump’s aides, including his former national security adviser, campaign chairman and personal lawyer, who have pleaded guilty or been convicted by a jury.

To protect those secrets from prying ears, the whole of the office suite in southwest Washington has been designated a Sensitive Compartmented Information Facility (SCIF), U.S. spy speak for an area that has restrictions to ensure secret information stays secure.

One common restriction in SCIFs is to keep out smartphones and other electronic devices, which can be turned into covert listening devices or spy cameras. Visitors also have been required to turn these over before entering.