A presidential historian charged with stealing historical documents and conspiring to take them from state archives in several states will remain in federal custody over the weekend, but a judge allowed his assistant to be released Friday.

At a hearing Monday, a judge will consider the prosecutors' recommendation that Barry Landau, 63, remain in custody. This will allow pretrial officials time to review his case and interview Landau, who appeared in the same blue-and-white-striped, button-down shirt and khakis that he wore to a bail review hearing in Baltimore Circuit Court earlier this week.

Federal prosecutor James G. Warwick said in court Friday that Landau poses a flight risk and might try to access documents that investigators haven't yet found and destroy evidence. Investigators believe that Landau has tried to tamper with witnesses, Warwick said, but he did not want to disclose details.

Prosecutors are looking at additional federal charges, including mail and wire fraud, interstate transportation of stolen goods and theft of government property, Warwick said.

U.S. Magistrate Judge Susan Gauvey approved an agreement Friday that allows Landau's assistant, Jason Savedoff, 24, be released to his mother on $250,000 cash bail. Savedoff, who appeared in a yellow jumpsuit from the Baltimore jail with a slight beard, has surrendered his American and Canadian passports and will stay at an apartment in the Baltimore area.

The men were arrested July 9 after a Maryland Historical Society employee reported Savedoff took a document out of the society's Baltimore library. When police arrived, investigators found 60 documents inside a library locker Savedoff was using, including papers signed by President Abraham Lincoln worth $300,000 and presidential inaugural ball invitations and programs worth $500,000, Baltimore prosecutors have said.

Landau had signed out many of those documents, according to court documents.

The two men were indicted by a federal grand jury Thursday, accused of stealing and selling historical documents that included a Benjamin Franklin letter and speeches by President Franklin D. Roosevelt.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 an April 1780 letter from Franklin to John Paul Jones from the New-York Historical Society in March. They are also charged with taking a set of signed inaugural addresses from the FDR presidential library in December and later selling them for $35,000.

U.S. Attorney Rod J. Rosenstein has said a nationwide investigation is continuing and encouraged anyone with information about the acquisition or sale of historical items by the two to contact the FBI. Special Agent in Charge Richard A. McFeely called "the scope and notoriety" of the documents seized in this case "truly breathtaking."

Before his arrest, Landau appeared on TV programs and was quoted in news articles, particularly for his knowledge of White House social events and drew upon his extensive collection of souvenirs to write a coffee-table book, "The President's Table: Two Hundred Years of Dining and Diplomacy."

The investigation included a search of the author's museum-like, New York City apartment that's lined with mementos dating back to Washington's presidency, the FBI said. Black-and-white etchings of 19th-century inaugurations hang from the walls, while a cabinet displays presidential goblets, plates and a skeleton key that purportedly fit the front door of the White House during John Adams' administration, according to a 2007 Associated Press article.

In the wake of the arrests, institutions across the country are reviewing their vulnerabilities. They often have limited money and must balance security measures against giving access to the public.