Though over a century old, one often overlooked aspect of government surveillance continues to expand and evolve: the tracking of Americans' mail by the U.S. Postal Service. Highlighting a "little-noticed" 2014 audit of the mail system in conjunction with a number of interviews and documents obtained under the Freedom of Information Act, a New York Times investigation published Monday reveals the scope of government monitoring of "snail mail."

According to the 2014 Inspector General audit, in 2013 the USPS approved nearly 50,000 requests for surveillance from law enforcement agencies and its own internal inspection unit, the Postal Inspection Service, to monitor the mail of Americans for use in criminal and national security investigations.

The surveillance program, officially known as "mail covers," permits the scrutiny of information on the outside of letters and packages before they are delivered to a person's home at the request of state or federal law enforcement agencies. This is in addition to a program called "mail imaging," under which the USPS photographs and stores the exterior information from every package and letter sent through their system.

Whereas opening mail requires a warrant, this "deceptively old-fashioned method of collecting data provides a wealth of information about the businesses and associates of their targets, and can lead to bank and property records and even accomplices," the Times reports.

The Times investigation found that from 2001 through 2012, local, state and federal law enforcement agencies made over 100,000 requests, amounting to roughly 8,000 requests a year.

Compared with requests reported in 2013, this number does not include requests made for national security investigations or those requested by the Postal Inspection Service. This substantial disparity, Times reporter Ron Nixon notes, "shows that the surveillance program is more extensive than previously disclosed."

The IG audit further revealed that "in many cases the Postal Service approved requests to monitor an individual’s mail without adequately describing the reason or having proper written authorization," writes Nixon, who added that this information shows that "oversight protecting Americans from potential abuses is lax."

Theodore Simon, president of the National Association of Criminal Defense Lawyers, told Nixon he was troubled by the audit. "It appears that there has been widespread disregard of the few protections that were supposed to be in place," Simon said.