"It's been determined that among the papers there are a number of classified U.S. government documents," FBI spokesman Bill Carter told The New York Times. "Under the law, no private person may possess classified documents that were illegally provided to them. These documents remain the property of the government."
But Anderson's son Kevin, a Salt Lake City lawyer, said family members were willing to go to jail to protect the collection.
"It's my father's legacy," Anderson said. "My father's view was that the public is the employer of these government employees and has the right to know what they're up to."
Anderson's files were stored for years at Brigham Young University before being transferred to George Washington University in Washington at Anderson's request last year, before he died of Parkinson's disease last December at age 83.
The FBI had expressed no interest in the notes until then, the Times said.
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