Michael Rohana, from Bear in the state of Delaware, was at the Science After Hours: Ugly Sweater Party on 21 December at Philadelphia's The Franklin Institute after the museum had closed for the day.
It was at this party that he decided to go into the institute's Terracotta Warriors of the First Emperor display while yielding his flashlight on his phone, The New York Times reported.
Rohana took a selfie with the 2000-year-old cavalryman, which is worth $5.7 million, before allegedly making off with its thumb and stored it in his bedroom desk drawer for almost a month.
Investigators connected him with the crime after Rohana left fingerprints at the scene, according to an affidavit filed by the FBI cited by The Inquirer.
Rohana was charged with theft, concealing artwork and transporting stolen goods across state lines last week before being released on bail.
The alleged theft has angered Chinese authorities with Shaanxi Cultural Heritage Promotion Centre director Wu Haiyun "strongly condemning" the institute for its "careless" approach to the statues.
The Shaanxi Cultural Heritage Promotion Centre was the government-run organisation behind the 10-statue loan to the institute.
“We ask that the US severely punish the perpetrator. We have lodged a serious protest with them,” Mr Wu told CCTV, according to the BBC.
The artworks featured in the institute's exhibition were found at the UNESCO World Heritage-listed Mausoleum of the First Qin Emperor in the Shaanxi Province.
The first emperor of unified China, Qin Shi Huang, is buried in the mausoleum surrounded by the iconic terrocotta warriors and horses, and bronze chariots and weapons.
Emperor Qin died in 210BC but the elaborate burial site was not discovered until 1974.
The Franklin Institute has since revised its security procedures and the thumb has been returned so the statue can be repaired, a spokeswoman told The Inquirer.
