WASHINGTON — Employees of the United States Postal Service failed to properly safeguard documents that included the names, addresses and financial information used by its law enforcement arm to monitor the mail of people suspected of criminal activities or for national security purposes, an internal investigation found.

The information, which is collected as part of the Postal Service’s mail cover surveillance program, could potentially reveal personally identifiable information and compromise the privacy of the mail, according to the report, which was conducted by the Postal Service’s Office of Inspector General and released Thursday.

A mail cover is a surveillance tool used by the service to monitor the mail of a person suspected of criminal activity by recording the information on the outside of all letters and packages delivered to a home or business. Law enforcement officials say it is an important investigative tool, but privacy advocates say the practice is ripe for abuse because it does not have judicial oversight and is shrouded in secrecy.

The report released Thursday follows a similar audit last year that examined the use of mail covers by outside law enforcement agencies.