The 3,200-year-old mask of Ka-Nefer-Nefer, a 19th Dynasty noblewoman, sits on display in the basement of the museum.
The federal complaint contends the mask was stolen from Egypt before the museum obtained it for $500,000 in 1998.
The complaint, which included a request for a restraining order preventing the museum from disposing of the mask during the legal proceedings, came a month after the museum sued the government to try to block the seizure of the mask.
U.S. Attorney Richard Callahan said the dispute was “unfortunate” and would be “resolved by the courts.”
The museum was not available for comment.
In its lawsuit, the museum said that the government did not have enough evidence that the mask was stolen and that the statute of limitations had run out on the government’s right to seize the mask.
The federal complaint said the government was “certain” the mask was stolen and had traced its path following its discovery by an Egyptian excavator in 1952. The mask was stolen sometime between 1966 and 1973, the complaint said.
In 2006, the Egyptian Supreme Council of Antiquities found out the mask had been purchased by the St. Louis museum and began its attempt to recover the piece.