WASHINGTON - Johnnie Burton, a former director of the Wyoming Department of Revenue, will retire as director of the federal Minerals Management Service, Interior Secretary Dirk Kempthorne announced Monday.

"I have served this president and the secretary of Interior for over five years," Burton said in a statement. "This has been the most rewarding, and often challenging, position of my career."

Burton was appointed to the federal post in March 2002. Her retirement will be effective at the end of this month.

Burton directed the Wyoming Department of Revenue from 1995 to 2002 and represented Natrona County in the Wyoming House from 1982 to 1988.

The Minerals Management Service is responsible for calculating royalties for federal minerals. In recent months, the agency has been criticized for poor auditing, mismanagement and retaliation. Critics say those problems have cost taxpayers billions of dollars.

In particular, Burton was criticized by the Interior Department's inspector general for failing to take action when she first heard of billion-dollar errors with oil and gas drilling leases.

Deep-water leases signed in 1998 and 1999 during the Clinton administration omitted a clause triggering royalty payments if energy prices rose over a certain amount. Officials say they raised the matter with Burton in early 2004, although she does not remember being told until late 2005 or early 2006, the inspector general found.

The error has already cost the government about $1 billion in revenue and if not fixed could cost $10 billion.

Burton testified before a House committee in September that she first learned about the issue in late 2005 or early 2006. But a former associate director said he was sure he had briefed Burton on the issue in 2004, the inspector general's report said.

Rep. Darrell Issa, R-Calif., who last year chaired the subcommittee that investigated the leases and heard Burton's testimony, suggested Burton should resign if she learned of the problems earlier than she told Congress. Burton defended herself at the time.

"I am now being held responsible for not remembering a passing mention of an issue in 2004, and for not doing anything to remedy a mistake made by the Clinton administration when, in fact, I wrote every company in June 2006 asking them if we could agree to an amendment to their leases," Burton wrote.