The mask from the St. Louis Art Museum is from the Egypt's 19th Dynasty ca. 1250 BC and is its star attraction.The mask was found at Sakkara in 1952, the Cairo Egyptian Museum went looking for it and found it missing from its holdings.
Kanefernefer's mask is worth at auction from about a half a million to a million dollars today, the Egyptian authorities have presented to the directors of the SLAM their evidence including photos of the mask emerging from its burial.
The Slam's directors reject Egypt's claim on the mask. The documents have been presented by Egypt to the international court as well as the International Art Theft Registry. The Egyptian Supreme Council of Antiquities is out to get you St. Louis, now you see it now you don't.
Very good article pulling apart the various provenances and explanations of St Louis's nineteenth dynasty mask and a mask found in the early 1950's and called by its excavator Kanefernefer which is missing.