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DENVER — The U.S. Department of Justice says it has lost track of more than 60 boxes of documents from a criminal investigation into environmental violations at a former nuclear weapons plant in Colorado.

The files are at the center of a court battle between the government and activists who want the documents made public. They have remained secret since a grand jury investigation into the Rocky Flats plant outside Denver ended with criminal charges 27 years ago.

The U.S. Attorney’s Office in Denver told the activists in a July 24 email it can’t find the files. The email was made public in a court filing Tuesday.

The activists say the files could show whether the government did enough to clean up Rocky Flats before turning part of it into a wildlife refuge.