The U.S. Marshals Service has lost track of at least 2,000 encrypted two-way radios and other communication devices valued at millions of dollars, according to internal agency documents, creating what some within the agency view as a security risk for federal judges, endangered witnesses and others.

The problem, which stretches back years, was laid out in detail to agency officials at least as early as 2011, when the Marshals were deploying new versions of the radios they use to securely communicate in the field. Agency leaders continued to have difficulty tracking their equipment even after they were warned about the problems by an internal technology office, according to the documents, which were obtained through Freedom of Information Act requests.

Some Marshals officials told The Wall Street Journal that besides the wasted money and resources, the inventory problems raise the possibility that criminals could get their hands on radios and listen to them to learn details of security or law-enforcement operations. Such radios are a key communications tool of U.S. Marshals.

USMS spokesman Drew Wade said the agency believes "this issue is in large part attributable to poor record keeping as a result of an older property-management system, as opposed to equipment being lost."

The Marshals Service guards judges and federal courthouses, and it runs the Witness Security Program, which provides new identities and security to witnesses or their families at risk of being killed. The Marshals also seek to apprehend fugitives.