US presidential historian Barry Landau, known for his huge collection of memorabilia, pleaded guilty to conspiracy and the theft of dozens of historical documents, US officials said.

Barry Landau, 63, admitted he and an assistant, 24-year-old Jason Savedoff, swiped items from museums in Maryland, Pennsylvania, New York and Connecticut, and sold selected documents for profit, according to a Department of Justice statement.

The pair face up to five years in prison for the conspiracy and 10 years for the theft. Savedoff previously pleaded guilty to the same charges.

In Landau's New York apartment, authorities uncovered over 4,000 items traced as being stolen from libraries and museums throughout the United States -- include documents signed by George Washington, Franklin Roosevelt, Marie Antoinette, Napoleon Bonaparte, Karl Marx, and Sir Isaac Newton, and others.

The pair were originally indicted last year with the theft of dozens of historical documents, in a spree from December 2010 through July last year.

Landau admitted he researched items to steal and the pair had methodically targeted the repositories and libraries "posing as researchers" for access, said the Justice Department in a statement.

Among the documents Landau and Savedoff stole was a letter dated April 1, 1780, from Benjamin Franklin, one of the Founding Fathers of the United States, to American Revolutionary War naval fighter John Paul Jones, which they stole from the New York Historical Society.

The pair also snatched some 60 documents from the H. Furlong Baldwin Library at the Maryland Historical Society in July, taking some 60 documents, including a land grant dated June 1, 1861, to a soldier from the Maryland Militia who fought in the War of 1812, and signed by president Abraham Lincoln.

That document was valued in excess of $100,000, officials said.

Landau, who claimed to be "America's Presidential Historian," has made regular TV appearances discussing presidential issues. His website claims "personal relationships" with presidents and their families, and shows him photographed with five US presidents.

His collection of presidential memorabilia contained over a million items, including 26,000 presidential menus and invitations, according to the website.

Landau's sentencing was scheduled for May 7, but no date has been set yet for Savedoff's sentencing.