The Palace Museum in China's historic Forbidden City is under scrutiny after admitting one of its workers broke a priceless centuries-old relic.

The museum says a 1,000-year-old piece of porcelain was destroyed in early July when a laboratory researcher used a machine to run tests on it. The porcelain, valued at more than $15 million, was made during the Song Dynasty, which began in 960 and lasted three centuries.

The destruction of the artifact was first reported by a Chinese blogger on July 30, and confirmed by the museum the next day. Chinese critics say the incident reflects a lack of basic professionalism required to operate a museum, noting that many staffers have had no formal training.

The Palace Museum is also under fire over a report that it bought five rare letters from the Song Dynasty in 1997, only to have the letters show up again on the auction market in 2005. Museum officials say they never had the letters to begin with.

In May, a man was arrested after managing to steal more than $1 million worth of art pieces from the museum. The thief, who was seen on surveillance cameras, is believed to have hidden inside the museum before it closed and escaped during the night.