The Justice Department has misplaced more than 60 boxes of documents from a 27-year-old criminal investigation into safety violations at a onetime nuclear weapons plant in Colorado, officials said Tuesday.

The files were gathered during a two-year grand jury probe of the Rocky Flats plant, which is about 15 miles northwest of Denver.

The plant, which made plutonium triggers for nuclear warheads during its nearly 40 years in operation, was prone to fires, leaks and spills. It shut down in 1989 amid a grand jury investigation. Files from that investigation have remained secret since the investigation ended with criminal charges in 1992.

In January, seven groups representing environmentalists, former nuclear workers, nearby residents and public health advocates filed a motion in federal court requesting that the files be made public.

The groups said the files could show if the government did enough to remedy the problem before turning part of the location into a wildlife refuge and opening it to hikers and bicyclists.

The U.S. Attorney's Office in Denver told the activists in an email last Wednesday that the files have been misplaced. Jeff Dorschner, a spokesman for federal prosecutors, confirmed that the office had sent the email.

The activists will ask a federal judge on Wednesday to order Justice to find the documents within 30 days.

Rockwell International, the contractor that operated the plant, pleaded guilty to criminal charges that included mishandling chemical and radioactive material. The company was fined $18.5 million.

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The government spent $7 billion cleaning up two square miles at the center of the site where the plutonium triggers were built. The former buffer zone around the plant, covering eight square miles, became Rocky Flats National Wildlife Refuge and opened to the public last fall.

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In a separate case, activists filed a federal lawsuit saying the government did not do enough to make sure the refuge is safe. That lawsuit is pending.

The Associated Press contributed to this report.