An ornate silver pen used by statesman John Hay to sign the Treaty of Paris in 1898 is missing from its display case in the Smithsonian Institution's popular "We the People" exhibit.

FBI investigators said the pen -- whose monetary value was not estimated by Smithsonian officials -- apparently disappeared sometime Monday after it had been taken from its usual niche on the second floor of the Smithsonian's Museum of American History to be photographed for inclusion in a scheduled book about the museum.

"Logic would point toward an inside job of some kind," said FBI special agent Larry Knisley. But he added that there are no suspects in the theft -- the first of a Smithsonian artifact since September 1979 when a $125,000 snuff box was reported stolen from the National Collection of Fine Arts. The 1979 incident was described at the time as the biggest theft in the history of the museum.

While it is "of great intrinsic value," museum spokesman Larry Taylor said, the 7 1/4-inch pen with its silver handle of rococo scroll design and a steel Esterbrook tip would be "virtually worthless to a thief, because it has no inscription or accompanying documentation to describe its historical significance." Taylor said that documentation remains at the Smithsonian, which received the pen as a gift in December 1969 from a descendant of Hay.

Hay was secretary of State in the McKinley administration at the time he signed the treaty.

"It's a rare kind of pen, but what makes it valuable is that it was used by Hay to sign the treaty that ended the Spanish-American War [but one] can't tell that by looking at it, though," said Taylor, who guessed the silver content alone would fetch "not even $1,000."

When the pen was removed from its green leather case to be photographed Monday, Taylor said, the photographer, hired by the New York publishing house of Harry N. Abrams Inc., was accompanied by political history division curator Herb Collins and members of his staff. The pen was thought to be securely in its regular display case Monday night, Taylor said, but was discovered to be missing Tuesday morning by a technician on Collins' staff.