(CNN) Chinese authorities are livid over the theft of a thumb from a 2,000-year-old terracotta warrior on loan to a US museum, calling for the perpetrator to be "severely" punished.

The ancient statue is valued at $4.5 million and considered a "priceless part of China's cultural heritage." It was on display at Philadelphia's Franklin Institute when the vandalism took place.

"We call on the United States to severely punish those who have done [this]," Shaanxi Provincial Cultural Relic Exchange Center's director told the Beijing Youth Daily, a state-run Chinese newspaper.

The warrior statues date back to 209 B.C., when the Qin Dynasty ruled China. The life-size statues were constructed to guard the tomb of China's first emperor, Qin Shi Huang. Now a UN world heritage site , they were discovered by a Chinese farmer in 1974 in Shaanxi province, according to the Franklin Institute's exhibit page.

Photos: Xi'an's Terra-cotta Army – Excavation of the world's largest underground army started after local farmers discovered the first Terra-cotta warrior while digging a well in 1974 in Xi'an, China. Hide Caption 1 of 10 Photos: Big dig – Pit 1 is the largest and most famous excavation site of terra-cotta soldiers. Some 6,000 warriors have been discovered here, but the majority remain unearthed. Hide Caption 2 of 10 Photos: Royal work – The army is only party of a garrison in the mausoleum of Qin Shi Huang, the ruler who first unified various Chinese states in 221 BC. Experts say the Terra-cotta Army showcases the high level of craftsmanship in play 2,200 years ago. Hide Caption 3 of 10 Photos: Ancient arms – The most famous warrior statue depicts a crossbow soldier. It was excavated from Pit 2 in 1994. Hide Caption 4 of 10 Photos: Exquisite carving – Details were painstakingly sculpted by ancient craftsmen -- even fingernails can be clearly seen. Hide Caption 5 of 10 Photos: Prestige pit – Pit 3 is the smallest among the three Terracotta Army pits, but it houses the highest-ranked commanders of the army, as well as war chariots. Hide Caption 6 of 10 Photos: Selfish emperor? – Not everyone has the same reaction to the figure. Fiona Gazzola, a 15-year-old British girl on a school trip, told CNN she found the warriors chilling. "It made me realize how powerful the emperor was, and he was quite selfish," she said. "He built all these warriors for the afterlife just for himself, and it cost a lot of people's lives and a lot of time." Hide Caption 7 of 10 Photos: More to come – Excavation and restoration of terra-cotta figures is ongoing. In coming years, hundreds of warriors are expected to be restored from pottery pieces dug from the museum's three main pits. Hide Caption 8 of 10 Photos: Not just soldiers – Near the Terra-cotta Army, Chinese archaeologists discovered hundreds of pits with buried funeral treasures. This bronze chariot excavated in 1980 highlights the metallurgical technology in the Qin Dynasty. It's believed it was meant to serve as the emperor's ride into the afterlife. Hide Caption 9 of 10 Photos: Each year, millions visit the Terra-cotta Army. It's located in Lintong, 25 miles from the Xi'an city center. The entire area benefits from tourism, which generated 4.6 billion yuan ($720 million) in 2014, according to the local government. Hide Caption 10 of 10

Delaware-resident Michael Rohana was arrested last week, US officials said, accused of stealing the thumb in December during an ugly sweater party hosted by the museum.

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