Years of major environmental cleanup work at Rocky Mountain Arsenal near Denver are over.

U.S. Interior Secretary Ken Salazar attended a ceremony Friday marking the milestone.

The arsenal has been turned into a wildlife refuge and Salazar and Hew Wolfe of the Army signed a document officially transferring 2,500 acres to the U.S. Fish and Wildlife Service for the refuge, which now covers more than 15,000 acres.

The $2.1 billion cleanup of the former chemical weapons manufacturing site finished a year ahead of schedule.

After World War II, Shell Chemical Co. made pesticides and other chemicals there until 1982. It was later designated a Superfund site before Congress in 1992 declared that it be turned into a national wildlife refuge.