The U.S. Postal Service almost never denies requests to track suspects’ mail on behalf of law-enforcement agencies through a controversial surveillance program known for having compliance problems, according to a federal auditor.

USPS Deputy Inspector General Tammy Whitcomb said in testimony for a House hearing  Wednesday that the Postal Service rejected only about 0.2 percent of the 6,000 outside requests last year for a practice known as mail covers.

The investigative technique involves recording information on the outside of individuals’ envelopes and parcels before the items are delivered, and then handing the data to law-enforcement agencies. It does not permit the opening of mail, which requires a search warrant.

The USPS inspector general’s office said in an audit report  this year that the Postal Service failed to follow key guidelines for mail covers, including recording and sending information after the orders had expired and neglecting to conduct annual reviews.

Additionally, about 20 percent of the requests from outside law-enforcement agencies were not approved by authorized personnel and that 13 percent were either unjustified or incorrectly documented, according to the report.

The Postal Service asked the inspector general not to release the findings publicly. The agency said disclosure would reveal “investigative techniques and related information which could compromise ongoing criminal investigations,” according to a management letter.

Politico first reported the findings in June, and the New York Times wrote a piece about them in October. But Whitcomb’s testimony marked the first time that the inspector general’s office publicly revealed that the Postal Service almost never denies mail-cover requests.

“Mail covers are an important law-enforcement tool, but adequate supervision is critical to ensure the protection of the public,” the deputy inspector general said in her testimony.

Law-enforcement agencies use the surveillance technique to track financial fraud, monitor drug trafficking and investigate other criminal activity.

Prince Georges County, Md., police Capt. Charles Hamby said in testimony for the hearing that his organization relied on mail covers to piece together the payment system for a narcotics-smuggling operation, identify a suspect’s alias and convict 14 individuals involved in the case. He described the Postal Service’s help as “critical to the success of the investigation.”

But Timothy Edgar, a former Obama White House privacy expert who now works with Brown University’s Watson Institute for International Studies, described the inspector general’s findings as troubling.

“They shake our confidence in longstanding principles of privacy and civil liberties,” he said in testimony for the hearing. “The Postal Service must be a stickler for proper procedure — it cannot afford to be lax.”

USPS management said in a memo responding to the inspector general’s audit that it agreed with the findings and planned to tighten up its procedures to address the concerns, including restricting the approval of mail-cover requests.

“All standard operating procedures will be reviewed, revised and adopted as practice,” the letter said.

Overall, the USPS authorized about 49,000 mail covers last year. Outside organizations issued about 22 percent of the requests, while the others were from internal agencies such as the Postal Inspection Service and the inspector general’s office, according to numbers Whitcomb provided in her testimony.

The House Oversight and Government Reform subcommittee that oversees the Postal Service held the hearing on Wednesday.