Landau and Savedoff were arrested July 9 at the Maryland Historical Society in Mount Vernon.
"They were too schmoozy to be regular people," Angerhofer said Tuesday, as society officials made their first extended comments about the theft.
Angerhofer said Landau and Savedoff arrived shortly after the library opened about 10:30 a.m. on a Saturday, and brought the staff cookies. Landau introduced Savedoff as his nephew. Though the library was busy in the morning, Angerhofer sensed something was amiss and decided to watch them "like a hawk."
He said Landau approached the circulation desk several times during the morning, acting as a "human screen" to block the two staffers' view of Savedoff. Angerhofer said he sent a colleague an email, acknowledging the odd behavior.
When Savedoff and Landau left for a lunch break, the library staffers dedicated themselves to getting "visual confirmation" of theft, Angerhofer said.
Angerhofer, who tried to stay out of view for much of the afternoon, said he watched from a balcony shortly before 4 p.m. as Savedoff slipped a society document under his own papers. The account is backed up in a police report.
"They acted very cool," Angerhofer said, describing their demeanor after he confronted them.
Pat Anderson, the society's director of publications and library services, said the staff does not believe any documents are missing, and there was no noticeable damage to the items Landau and Savedoff hid. None of the items ever left the building, she said.
Police said they seized 79 stolen documents — 60 taken from the society — hidden in a computer bag in a small coin locker in the reading room, to which Savedoff had the key.
On Tuesday, officials also showed two display cases with some of the presidential "ephemera" — dance cards from inaugural balls, admission tickets for the impeachment proceedings of President Andrew Johnson — that they said the two tried to carry out of the library.
The society is looking for funding to install security cameras in the reading room and has begun searching visitors' personal papers as they exit, Anderson said.
The investigation quickly expanded beyond Baltimore, as curators across the nation scoured their collections and reviewed visitor logs. More thefts were discovered, and librarians recalled the two men as suspicious because of their vague and shifting requests, unrelated to the topics they said they were researching.
New court documents filed Tuesday reveal an intricate scheme in which Landau and Savedoff carefully studied which documents they wanted — a determination usually based on worth rather than substance or relevance to a particular line of study.
Landau hit archives up and down the East Coast, according to the plea agreement, from the New York, Pennsylvania and Connecticut historical societies, to the University of Vermont and the Roosevelt library.
Items seized from his apartment have been turned over to the National Archives, where employees have been cataloging them and trying to track down owners. Although authorities recovered the three Roosevelt inaugural addresses from Landau's apartment, court documents say four other original Roosevelt documents were sold to a collector for $35,000.
Paul Brachfeld, the National Archives' inspector general, said in an email last week that after Landau and Savedoff are sentenced, he envisioned an unusual procedure for determining the ownership of "those items for which clear and legal provenance cannot be defined." He described it as "a reverse 'Antiques Roadshow' where the public could come to view the holdings and 'claim' [them] if they have the legal basis to do so."
Assistant U.S. Attorney James G. Warwick said in court on Tuesday that many vendors who bought items from Landau have returned them to the government, and that as part of his plea, Landau is to pay them back.
Prosecutors have said in court that they believe the document thefts have gone on for years, but the charges to which Landau pleaded guilty to are limited to the past two years. Landau's defense attorney disputes one contention by authorities, that Landau ordered Savedoff to steal documents.