At the hearing, postal officials sought to assure members of the panel that the agency had adequate controls over the surveillance to protect the privacy and civil liberties of Americans.

Guy Cottrell, chief of the Postal Inspection Service, the law enforcement arm of the Postal Service, said the agency had made several changes in the way it managed the mail covers program after the audit in May. The audit found that 21 percent of law enforcement requests to monitor mail had been approved without written authorization from the proper Postal Service officials, and that 13 percent had lacked properly documented explanations for their approval.

The audit also found that the Postal Inspection Service had not conducted annual reviews of the mail cover program as required by federal law, and that more than 900 mail covers were still active even though the orders for them had expired.

“We had made substantial progress in addressing the issues raised in the audit,” Mr. Cottrell told the subcommittee. He provided data showing that the number of mail covers requested by law enforcement agencies had declined to about 6,700 in 2013 from slightly more than 9,400 in 2010.

But Mr. Cottrell said the Postal Service’s overall use of mail covers, including by its internal law enforcement arm, had jumped to about 57,000 so far this year from about 16,000 in 2012. He said the increase was due to the Postal Inspection Service’s use of one-day covers, mostly in cases of suspected drug smuggling through the mail. The one-day covers permit surveillance of a single piece of mail rather than of all the mail going to an individual or business.