The warrior was a symbol of martial strength, molded from terra cotta and buried more than 2,000 years ago with China’s first emperor to defend him in the afterlife.

The statue was helpless, however, against a man in a green sweater and a Phillies hat who, the authorities say, sneaked into a closed-off area during a party at the Franklin Institute in Philadelphia and stole its thumb.

The man, who was attending the museum’s after-hours ugly-sweater party on Dec. 21, entered the terra-cotta warrior exhibition room and used his cellphone’s flashlight to view the displays. Then, according to an affidavit by Jacob B. Archer, a special agent assigned to the F.B.I.’s art crime team, the man put his arm around the statue and took a selfie.