A presidential historian has been scheming to steal valuable documents from archives throughout the Northeast for years — if not decades — and his release from federal custody could put more pieces of American history at risk if he tries to cover his tracks, a prosecutor argued Thursday.

U.S. Magistrate Judge Susan K. Gauvey approved the release of Barry Landau, 63, to his Manhattan apartment with GPS monitoring, but put the release on hold until another judge hears an appeal from prosecutors Friday afternoon.

Landau and his assistant, 24-year-old Jason Savedoff, are charged with stealing valuable historical documents from the Maryland Historical Society and conspiring to steal documents from other archives. The historian would use different routines to distract curators and had sports jackets and overcoats altered to allow him to stash documents inside large pockets, Assistant U.S. Attorney James G. Warwick said.

The two New York men were arrested in the historical society's library in Baltimore last month after arousing employees' suspicions, according to court documents.

Landau pleaded not guilty on Thursday. Savedoff has yet to enter a plea.

Warwick told the judge that the pair had some 80 documents. About 60 were from the Maryland Historical Society, including papers signed by President Abraham Lincoln worth $300,000 and presidential inaugural ball invitations and programs worth $500,000. The other documents were from the Connecticut Historical Society, Vassar College and the National Archives, Warwick said.

Investigators have twice searched the apartment Landau shared with Savedoff but are concerned that if Landau is released he could destroy documents elsewhere, Warwick said. Landau bragged that he had a storage space in the Washington, D.C., area where he had 30 times the number of paintings, documents and artifacts that he kept in his apartment — which Warwick described as "wall-to-wall" memorabilia.

"If we don't get to them first, they may be lost forever," Warwick said.

Landau's attorney, Andrew C. White, said that his client doesn't have any other repositories for documents and memorabilia, and if he said as much, it was only to boost his image.

A search of Landau's apartment last month turned up thousands of documents, Warwick said. National Archives workers have been cataloging the documents, and have determined so far that 200 belong to institutions, including Swarthmore College, the Smithsonian Institution, Yale University, Columbia University, the New York Public Library, Vassar College, Cambridge University, University of Vermont and the Library of Congress.

"These are priceless relics of history," Warwick said. "They're no longer available to the public. They've been converted for Mr. Landau's personal financial gain."

Warwick offered a peek into a "complex bundle of lies and deceit" he said the government has been uncovering. Investigators were told that Landau would offer to reframe paintings he admired in people's homes, and then return high-quality reproductions he commissioned and keep the originals, Warwick said.

He also said Savedoff would use identification stolen from wallets at a New York gym to distance himself from Landau when they visited archives.

During a July 12 search of the apartment, investigators could only take documents they believed were stolen, but they photographed the items they left. They were later told Landau shredded historical documents that he had more than one copy of to enhance the value of the remaining documents, Warwick said.

When investigators returned Tuesday, Warwick said the shredder had been moved and cleaned out.

Landau's attorneys informed prosecutors that an attorney would remove some paintings to sell to pay for Landau's defense, but Warwick said other items, including photos of famed pilot Charles Lindbergh, were missing when investigators returned on Tuesday. There was also fresh paint on one wall, perhaps meant to disguise the removal of other items, he said.

Warwick said the historian, even while in jail, tried to get another person to take the blame for the thefts, offering to pay for a psychiatric defense. Landau made 91 phone calls while in state custody, and while Warwick said no evidence of obstruction was found during those calls, Landau also had visitors, the prosecutor said.

The men were indicted by a federal grand jury last week, accused of stealing and selling historical documents that included a Benjamin Franklin letter and speeches by President Franklin D. Roosevelt. They also face state theft charges.

The federal indictment charges the pair with stealing a Franklin letter from the New-York Historical Society in March; the letter was written to John Paul Jones in April 1780. Landau and Savedoff are also charged with stealing a set of signed inaugural addresses from the FDR presidential library in December and later selling some of them for $35,000. Some of those documents are still missing, Warwick said.

Savedoff was released last week on $250,000 cash bail and will stay at a Baltimore area apartment.