An archivist and an antique bookseller were charged Friday with stealing millions of dollars’ worth of rare books, illustrations, maps and photographs from a Pittsburgh library over a 20-year period, including Isaac Newton’s “Principia,” considered a watershed of science.

In some cases, the archivist would use an X-Acto knife to cut potentially valuable maps and illustrations out of books in the Carnegie Library of Pittsburgh’s historical collection, authorities said. They ultimately sold to other collectors famous originals that were hundreds of years old, authorities said.

Researchers found more than 300 items damaged or missing, a loss estimated at $8 million.

Charged were former Carnegie Library archivist Gregory Priore, 61, and bookstore owner John Schulman, 54, with theft, conspiracy, forgery, receiving stolen property and other counts in the disappearance of hundreds of items.

Lawyers for the men did not return messages seeking comment.

The alleged scheme started unraveling last year when appraisers with Pall Mall Art Advisors, a Pennsylvania-based art consultancy, began an audit sought by the library and discovered that items were missing or damaged since the last audit in 1991. 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, Caliban Book Shop, 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.

One particularly valuable item allegedly stolen and recovered was Newton’s “Philosophiae Naturalis Principia Mathematica,” with an estimated value of $900,000.

Listed as a total loss are: “An Inquiry into the Nature and Causes of the Wealth of Nations” by Adam Smith, at a value of $180,000; “The Journal of...” by George Washington, estimated at $250,000; and “Reise in das Innere Nord-America” by Prince Alexander Philipp Maximilian and Karl Bodmer, at a value of $1.2 million.

In one case, 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.

A Pall Mall appraiser helping detectives found that book, “De la France et des Etats-Unis,” for sale online for $95,000 from a bookstore in New York. The book had been bought and sold twice after Schulman allegedly sold it to Bartleby’s, detectives said.

In a 36-page narrative filed in court, Priore told detectives that he first approached Schulman in the late 1990s about selling items from the library’s special collection and that Schulman agreed to do it.

Priore, who in 1991 became manager and archivist of the library’s special collections room, simply walked out of the library with books, authorities say. Sometimes he concealed maps or illustrations in a manila folder or, if it was larger, rolled it up and carried it out, they said.

“Priore chose items from the Oliver Room to give to Schulman to sell if Priore felt they had value,” the complaint said. “He was unable to recall exactly what he took because he would just grab stuff off of the shelf.”

Sometimes, Schulman requested a specific item from Priore, authorities said. Priore removed potentially valuable maps and bookplates from books that were otherwise in poor condition, even using an X-Acto knife to cut some items out, they said.

Schulman paid Priore up front and pocketed the cash from items he could unload, detectives said. Priore estimated that he netted $40,000 to $50,000 in total, and suspected that Schulman had received much more, detectives said.

The Carnegie library system was founded over a century ago with $1 million from steel magnate Andrew Carnegie, who went on to establish over 2,500 libraries around the world, according to the library.