WASHINGTON (AP) -- An Oregon congressman called Monday for an investigation into how the U.S. Forest Service allowed 16 acres inside a rare tree reserve to be logged as part of a salvage harvest following the 2002 Biscuit fire.

The tree-cutting inside the 350-acre Babyfoot Lake Botanical Area in southwestern Oregon was discovered by environmentalists last month, after an approved timber sale was completed.

The Forest Service has said employees of the Rogue River-Siskiyou National Forest mismarked the area where the logging took place, although just who did it or how the mistake happened has not been determined. Normally trees are marked with stapled tags and paint to show the boundaries of timber sales and reserves within them.

"Given the large size of the illegal harvest, the relatively clear boundary (a steep ridgeline and road) ... I find it difficult to understand how this could have been a casual oversight," said Rep. Peter DeFazio, D-Ore.

DeFazio asked Phyllis Fong, inspector general of the U.S. Department of Agriculture, to determine who approved expansion of the so-called Fiddler timber sale into the Babyfoot Lake Botanical Area; when the decision was made; and how the decision was reached. The Forest Service is part of the USDA.

DeFazio also asked whether the Rogue River-Siskiyou forest has adequate staff and funding, and what actions should be taken to ensure future mistakes are avoided.

Forest Service officials have called the improper logging "a serious error" and vowed to look into how the mistake was made.

Spokeswoman Patty Burel said she had not seen DeFazio's letter and could not comment on it. Burel blamed the logging error on a lack of communication and inadequate oversight, but said the agency is "committed to looking at this situation and preventing it in the future."

Barbara Ullian, conservation director of the Siskiyou Project, an environmental group that discovered the damage, said the incident demonstrated the importance of allowing the public to monitor logging operations in national forests.

The Forest Service closed the area to the public in March after protesters attempted to block logging roads and sit in trees.

The Siskiyou Project counted 290 stumps inside the botanical area, including one that measured 3 feet in diameter and was more than 230 years old.

The Biscuit fire, the largest in Oregon history, burned more than 500,000 acres in July 2002. The fire threatened 17,000 people and cost $153 million to control.