DENVER (AP) - The U.S. Department of Justice has lost track of more than 60 boxes of documents from a 27-year-old criminal investigation into safety and environmental violations at a former nuclear weapons plant in Colorado, officials said Tuesday.

The files were gathered in a two-year grand jury probe of the Rocky Flats plant outside Denver, which manufactured plutonium triggers for nuclear warheads and had a history of fires, leaks and spills.

The files have remained secret since the investigation ended with criminal charges in 1992.

Seven groups representing environmentalists, former nuclear workers, nearby residents and public health advocates filed a motion in federal court in January asking that the files be made public. The groups say the documents could show whether the government did enough to clean up the site before turning part of it into a wildlife refuge and opening it to hikers and bicyclists.

Government attorneys are fighting the request.

The U.S. Attorney’s Office in Denver told the activists in a July 24 email that it cannot find the files.

Kevin Traskos, chief of the U.S. attorney’s civil division in Denver, told The Associated Press on Tuesday that officials have done only an initial search so far. “The search was preliminary and is continuing,” he said.

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

Rocky Flats, perched on a windy plateau about 15 miles (24 kilometers) northwest of downtown Denver, operated from 1952 to 1989. It shut down amid the grand jury investigation.

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.

The government spent $7 billion cleaning up 2 square miles (5 square kilometers) at the center of the site where the plutonium triggers were built. The former buffer zone around the plant, covering 8 square miles (21 square kilometers), became Rocky Flats National Wildlife Refuge and opened to the public last fall.

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

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This story has been corrected to show the activists have not yet asked a judge to order the Justice Department to find the files but plan to submit the request Wednesday.

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Follow Dan Elliott at http://twitter.com/DanElliottAP.

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