BALTIMORE (AP) – A memorabilia collector and self-styled expert on presidential history pleaded guilty Tuesday to conspiring to steal thousands of documents signed by leaders throughout U.S. history.

Barry Landau, whose knowledge of the White House earned him network morning show appearances, acknowledged in the plea to taking documents from the Maryland Historical Society and conspiring with his assistant to steal historical documents from several institutions with the intent of selling them.

Thousands of documents were seized from Landau’s artifact-filled Manhattan apartment. Prosecutors say he schemed for years, if not decades, to steal valuable documents signed by historical figures from both sides of the Atlantic including George Washington, Abraham Lincoln, Marie Antoinette and Charles Dickens. The oldest document listed in the plea was dated 1479.

The assistant pleaded guilty in October to the same charges: theft of major artwork and conspiracy to commit theft of major artwork. The pleas capped a case that was a wake-up call for archives and historical institutions nationwide to strengthen their security, prompting checks for visits by the pair and whether anything from historical collections was missing.

David S. Ferriero, archivist of the United States, said in a statement Tuesday evening that, “I am outraged that Mr. Landau who fashioned himself as a presidential historian violated the public trust at many of our nation’s greatest historical repositories.”

Landau, 63, and Jason Savedoff, 24, were arrested last July in Baltimore after alert Maryland Historical Society staffers realized something was off about the pair who plied staffers with cookies and portrayed themselves as uncle and nephew.

“They were too schmooze-y to be regular people,” David Angerhofer, a library archivist working that day. The collections the pair requested—coupled coupled with Landau’s attempts to block their view of Savedoff during repeated chats with staffers—made them suspicious and Angerhofer hid in the balcony for a better view.

After spotting Savedoff slip a document into a portfolio with personal papers, Angerhofer confronted Savedoff, saying he’d need to check the portfolio and that police were on their way. “I think they saw we were a small operation and they thought they had a golden opportunity to rob us blind,” Angerhofer said.

The pair had 79 documents hidden in a computer bag when they were arrested, according to Landau’s plea. About 60 of those belonged to the Maryland society, including a land grant signed by President Abraham Lincoln worth $300,000 and presidential inaugural ball invitations and programs worth $500,000.

There were questions about whether Savedoff had flushed away documents during a trip to the bathroom after staff stopped him, but nothing could be proved, according to Pat Anderson, director of publications and library services at the historical society.

According to Landau’s plea agreement, he stole historical documents from museums in Maryland, Pennsylvania, New York and Connecticut, and sold some for profit. The pair stole seven copies of speeches Roosevelt’s speeches from the Franklin D. Roosevelt Presidential Library in New York in December 2010, four of which Landau sold weeks later for $35,000, according to the plea. Each could have sold for more than $100,000.

To conceal thefts, Landau and Savedoff often took the card catalog entries and other “finding aids,” making it difficult for museums to discover that an item was missing, according to the court documents. In his plea, Landau disputes that Savedoff was acting at the older man’s direction when he committed the crimes.

In the weeks after Landau’s arrest, the FBI seized 10,000 documents and objects of cultural heritage from his apartment. So far, investigators had traced more than 4,000 of those to libraries and other repositories, according to Landau’s plea.

The pair compiled lists of historical figures, often noting the market value of documents signed by them, and Savedoff identified collections with valuable documents that they could target, according to the plea.

A weary-looking Landau appeared in a suit at Tuesday’s court hearing wearing an eye patch and using a cane. He listed the various medications he was taking for the judge, but noted that none was affecting his ability to understand the proceedings.

Landau had portrayed himself as an expert on presidential history and etiquette, and he was quoted in articles and interviewed on television programs.

A 2007 Associated Press article, written when Landau’s book, The President’s Table: Two Hundred Years of Dining and Diplomacy was published, includes his tale of how his fascination with the presidency began. Landau said at age 10 he parlayed a meeting with President Dwight Eisenhower during an appearance in New York into an invitation to the White House.

Landau faces up to 10 years in prison at sentencing on May 7. He may also have to pay restitution. No date is set for Savedoff’s sentencing hearing.

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