ST. LOUIS — The St. Louis Art Museum will keep a 3,200-year-old mummy mask unless it gets more proof that it belongs to Egypt.

The museum won't meet a Monday deadline set by Egyptian antiquities authorities to return the mask, museum director Brent Benjamin said Friday. He noted that the Supreme Council of Antiquities never officially gave the museum a deadline.

Zahi Hawass, secretary general for the Supreme Council of Antiquities in Egypt, said that the mummy mask was probably stolen before it was obtained by the art museum in 1998.

"Nothing that we have seen to this date supports his claim," Benjamin said.

Hawass gave the museum some documentation, including a register that recorded the burial mask of Ka Nefer Nefer being sent to the Egyptian Museum in Cairo, Egypt, in 1959.

Hawass has been critical of the museum for not returning the mask and has threatened to turn the dispute over to authorities. He also has threatened to tarnish the museum's reputation.

"Either provide us with the documentation," Benjamin said, or end the attacks on the museum.

The museum bought the mask from an art dealer in the United States in 1998 for about $500,000, only after checking with authorities and the international Art Loss Register to see if the item was stolen. The museum also approved the purchase with the Egyptian Museum, Benjamin said.

The museum has a money-back guarantee with the dealer, if the mask has to be returned to Egypt.

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The burial cover made of wood and plaster with glass eyes for the Egyptian woman was excavated from a pyramid in 1952 in Saqqara, Egypt. Hawass said because of poor record-keeping practices, it was documented only once, in 1959.

Hawass said under no circumstances could the mask have reached the United States legally because it belonged to the Egyptian Museum and there is no indication that the museum ever sold it.

Hawass said the mask could have been stolen in the 1980s when an Egyptian Museum storage room was looted in Cairo.