WARSAW, Poland — Half of a historic Auschwitz-Birkenau barracks that was on loan to the U.S. has been returned to Poland after two decades and long negotiations, officials said Sunday.

The Nazi barracks was one of the main items at the U.S. Holocaust Museum in Washington, which wanted the lease extended. But Poland asked for it back after adopting new regulations in 2003 that limit the loan of all historical and art works to a maximum of five years. That led to years of negotiations between the museums and U.S. and Polish governments that ended in October, when the Holocaust Museum agreed to return its portion of the barracks.

The Auschwitz-Birkenau museum said on its website that the wooden structure arrived at Poland’s Baltic port of Gdynia on Sunday. The barracks will undergo conservation and be joined with its other half in Birkenau. The procedure may take up to three years, according to Rafal Pioro, deputy director of Poland’s Memorial and Museum Auschwitz-Birkenau.

The effort to reassemble the barracks at its original location indicates Poland’s determination to be a guardian to the authenticity and integrity of the world’s largest Holocaust memorial, said the museum’s director, Piotr M. A. Cywinski.