But prosecutors have said that many more documents were pilfered. Assistant U.S. Attorney James Warwick said at a hearing this month that investigators have identified hundreds of stolen documents from at least 11 institutions, and the scheme may have been going on for "years if not decades." Landau's apartment on West 57th Street, which he shared with Savedoff, has been described as museum-like, with historic photos, artifacts and documents filling the walls.

Since the news of the arrests in Baltimore, staff members at archives across the country have been remembering visits by Landau, either alone or in the company of Savedoff. Landau, who maintains a website that identifies him as "America's presidential historian" and who has written a book, "The President's Table: 200 Years of Dining and Diplomacy," was particularly memorable.

"He said he had dated President Nixon's daughter," said Morgan Davis, a former archivist at the Missouri Historical Society.

In retrospect, Davis said, there was something "kind of off" about Landau, but nothing so far out of the usual realm in a field that has its share of personalities.

"When you're dealing with archivists or historians, a lot of times they're kind of eccentric," said Davis, who remembers Landau calling at least four times in advance of his visit to the society's museum and archive inSt. Louis in March 2008. Sometimes, he would keep her on the phone for 20 or 30 minutes, chatting about the time he spent at the White House and his connections to presidents and celebrities.

"I'm kind of a sympathetic person," she said. "And honestly, it was kind of interesting."

Landau took the opportunity to visit the archive while he was at the St. Louis public library to sign copies of his book. Officials at the museum and archive said they have checked the collections Landau viewed and found nothing missing.

Other historical troves may not have been as lucky.

Federal investigators have said that along with the 60 items from the Maryland Historical Society that were found in a locker that the pair using were 20 documents from other institutions, including the Connecticut Historical Society.

And searches of Landau's apartment found even more documents, numbering in the hundreds, that investigators say were stolen from the National Archives in Washington and universities such as Yale and Columbia.

"We certainly are in good company when I see the institutions they visited," Malley of the Connecticut Historical Society said wryly.

Malley said the society's staff is reviewing collections checked out by Landau and Savedoff during their four visits, twice in January and twice in March, to the archive in Hartford. There, Landau in particular was "not shy" about portraying himself as an experienced researcher and collector, even advising the archive's staff how to do its job, Malley said.

"They were looking at a folder of menus, and [Landau] said, 'Here's a menu from the dinner of Lincoln's second inaugural. … It really is a valuable piece and it should probably be conserved,'" Malley said. "He felt he was an important character."

Malley wonders if that was a ploy to ingratiate themselves to the archive staff. As at other archives, Landau and Savedoff brought sweets — Pepperidge Farm cookies — to the staff.

Library staffers have been working with investigators, sharing records of when Landau and Savedoff visited, and matching what has been found with their inventories. In one sense, investigators could not have a better group to work with: Librarians and archivists are by definition meticulous, trained to catalog and preserve historical documents and artifacts. But by the same token, historical society staffers say, there is no way to monitor that every one of the millions of items in their collections makes its way back to storage.

"A box may contain 100 pieces of paper," Arnold said.

Something that appeared to be missing might turn up in a different folder or box, simply misfiled, he said. Or, it may have vanished long before Landau and Savedoff asked to view the collection of which it was a part.

What sets archives apart from museums, Arnold said, is that even an important document such as the Washington letter that went missing temporarily is kept in the context of a collection, rather than displayed in isolation on a wall or behind glass.

If there is a silver lining to the alleged thefts at the nation's historical repositories, it's that "our system worked in the end," said Burt Kummerow, president of the Maryland Historical Society.

"We had people who were on the ball," he said of the volunteer and paid staffers who spotted suspicious behavior on the part of Landau and Savedoff, and called police, halting what prosecutors charge was a long-running plot to steal history.

"It is a violation. It is a violation of the whole society," Kummerow said. "But in the end … librarians are tough characters."