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, an F.B.I. special agent, the man put his arm around the statue and took a selfie. The authorities said the man, later identified as Michael Rohana, then went for a more permanent memento.