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The U.S. government is demanding a San Francisco museum give up claims to two religious relics allegedly stolen from Thailand, the U.S. attorney’s office announced Tuesday.

A civil complaint filed Monday in federal court seeks the forfeiture of two 1,500-pound hand-carved sandstone lintels the government contends were looted from ancient temples in Thailand.

The lawsuit says the items illegally made their way to a private collector in the United States and were donated to the city- and county-owned collection of the Asian Art Museum.

Thailand began investigating after the Thai consulate general in Los Angeles saw the lintels on display when he visited the museum in 2016, according to the suit.

“Returning a nation’s cultural antiquities promotes goodwill with foreign governments and citizens, while significantly protecting the world’s cultural history and knowledge of past civilizations,” said a statement from Tatum King, special agent in charge with Homeland Security Investigations in San Francisco. “The theft and trafficking of cultural artifacts is a tradition as old as the cultures they represent.”

The museum said one lintel is from Nong Hong Temple and dates to 1000-1080 AD. The other is from Khao Lon Temple and dates to 975-1025 AD.

The museum says one item was bought by noted collector Avery Brundage and the other by the museum, with Brundage as a go-between, in the 1960s from sellers in London and Paris.

The museum said that its own study found no evidence that the lintels were looted but also didn’t turn up any copies of required export documents required under Thai law, so it took them off public display and was planning to return them.

The lawsuit is surprising because the museum had been negotiating with both the Department of Homeland Security and Thai officials since 2017, said Robert Mintz, the museum’s deputy director.

The lengthy process of permanently removing the items from the museum’s collection had been expected to be completed this spring but now ”the lintels won’t go anywhere until the legal process is complete,” he said.

“We’re surprised by this filing and we’re disappointed that it seems to throw up a roadblock to what seemed like positive and developing negotiations.” he added.