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PITTSBURGH, Pa. (CBS/AP) — Archivist Greg Priore and bookshop owner John Schulman were charged Friday with teaming and scheming to steal more than $8 million worth of books, maps and rare items from the Carnegie Library.

Investigators say the scheme happened over a period of 20 years.

Researchers found more than 300 items damaged or missing.

One of the rare books included Sir Isaac Newton’s “Principia” which is considered a watershed of science.

Gregory Priore. Credit: CBS3.

In some cases, the archivist used an X-Acto knife to cut potentially valuable maps and illustrations out of books in the Pittsburgh library’s historical collection, authorities said.

At another point, the men allegedly forged a letter from the library to help them sell a book signed by Thomas Jefferson. The letter was designed to make it look as though the library had removed the book from its archives, detectives said.

They ultimately sold to other collectors famous originals that were hundreds of years old, authorities said.

The alleged scheme started unraveling last year when appraisers began a routine audit commissioned by the library and discovered that items were missing or damaged since the last audit in 1991.

John Schulman. Credit: CBS3.

The library locked down the room, and appraisers quickly began finding missing items for sale online, as well as items that had been sold or advertised by Schulman’s bookstore, a block away from the library.

In June 2017, library officials contacted authorities and fired Priore.

Detectives say efforts to recover the items have netted books, plates and maps estimated at a value of $1.1 million. Some were found during a search last year of Schulman’s book warehouse, detectives said.

Priore, 61, and Schulman, 54, are charged with theft, conspiracy, forgery, receiving stolen property and other counts in the disappearance of hundreds of items.

Priore’s lawyer wouldn’t comment.

Both suspects are currently free.

Their preliminary hearings are set for August 1.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)