WASHINGTON — In a rare public accounting of its mass surveillance program, the U.S. Postal Service reported that it approved nearly 50,000 requests last year from law enforcement agencies and its own internal inspection unit to secretly monitor the mail of ordinary Americans for use in criminal and national security investigations.

The number of requests, contained in a little-noticed 2014 audit of the surveillance program by the Postal Service’s inspector general, shows that the surveillance program is more extensive than previously disclosed, and that oversight protecting Americans from potential abuses is lax.

The audit, along with interviews and documents obtained by The New York Times under the Freedom of Information Act, offers one of the first detailed looks at the scope of the program, which has played an important role in the nation’s vast surveillance effort since the terrorist attacks of Sept. 11, 2001. The audit found 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.

In addition to raising privacy concerns, the audit questioned the efficiency and accuracy of the Postal Service in handling the requests. Many requests were not processed in time, the audit said, and computer errors caused the same tracking number to be assigned to different surveillance requests. “Insufficient controls could hinder the Postal Inspection Service’s ability to conduct effective investigations, lead to public concerns over privacy of mail and harm the Postal Service’s brand,” the audit concluded.

The audit was posted in May without public announcement on the Postal Service inspector general’s website and got almost no attention.

The surveillance program, officially called mail covers, is more than a century old, but is still considered a powerful investigative tool. At the request of state or federal law enforcement agencies or the Postal Inspection Service, postal workers record names, return addresses and any other information from the outside of letters and packages before they are delivered to a person’s home. Law enforcement officials say 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. (Opening the mail requires a warrant.)

Interviews and court records also show that the surveillance program was used by a county attorney and sheriff to investigate a political opponent in Arizona — the county attorney was later disbarred in part because of the investigation — and to monitor privileged communications between lawyers and their clients, a practice not allowed under postal regulations.

Theodore Simon, president of the National Association of Criminal Defense Lawyers, said he was troubled by the audit and the potential for the Postal Service to snoop uncontrolled into the private lives of Americans. “It appears that there has been widespread disregard of the few protections that were supposed to be in place,” Mr. Simon said.

In information provided to The Times earlier this year under the Freedom of Information Act, the Postal Service said that from 2001 through 2012, local, state and federal law enforcement agencies made more than 100,000 requests to monitor the mail of Americans. That would amount to an average of some 8,000 requests a year — far fewer than the nearly 50,000 requests in 2013 that the Postal Service reported in the audit.

The difference is that the Postal Service apparently did not provide to The Times the number of surveillance requests made for national security investigations or those requested by its own investigation and law enforcement arm, the Postal Inspection Service. Typically, the inspection service works hand in hand with outside law enforcement agencies that have come to the Postal Service asking for investigations into fraud, pornography, terrorism or other potential criminal activity.

The Postal Service also uses a program called Mail Imaging, in which its computers photograph the exterior of every piece of paper mail sent in the United States. The program’s primary purpose is to process the mail, but in some cases, it is also used as a surveillance system that allows law enforcement agencies to request stored images of mail sent to and received by people they are investigating.

Another system — the Mail Isolation Control and Tracking Program — was created after anthrax attacks killed five people, including two postal workers, in late 2001, and is used to track or investigate packages or letters suspected of containing biohazards such as anthrax or ricin. The program was first made public in 2013 in the course of an investigation into ricin-laced letters mailed to President Barack Obama and former New York City Mayor Michael R. Bloomberg by an actress, Shannon Guess Richardson.

Despite the sweep of the programs, postal officials say they are both less intrusive than that of the National Security Agency’s vast collection of phone and Internet records and have safeguards to protect the privacy and civil liberties of Americans. “You can’t just get a mail cover to go on a fishing exhibition,” said Postal Inspection Service spokesman Paul J. Krenn. “There has to be a legitimate law enforcement reason, and the mail cover can’t be the sole tool.”

The mail cover surveillance requests cut across all levels of government — from global intelligence investigations by the U.S. Army Criminal Investigations Command, which requested 500 mail covers from 2001 through 2012, to state-level crimes by the Georgia Bureau of Investigation, which requested 69 mail covers in the same period. The Department of Veterans Affairs requested 305, and the State Department’s Bureau of Diplomatic Security asked for 256. The information was provided to The Times under the Freedom of Information request.

Postal officials did not say how many requests came from agencies in charge of national security — including the FBI, the Department of Homeland Security and Customs and Border Protection — because release of the information, wrote Kimberly Williams, a public records analyst for the Postal Inspection Service, “would reveal techniques and procedures for law enforcement or prosecutions.”

Defense lawyers say the secrecy concerning the surveillance makes it difficult to track abuses in the program because most people are not aware they are being monitored.