The U.S. Ambassador to Poland, Lee Feinstein, has announced that he will be ending his service.

In a sudden announcement on Thursday, the American Embassy in Warsaw said Feinstein would be retiring from his position and that Steven D. Mull, who already served in that role from 1993 to 1997 and was part of the negotiations team for adding Poland to NATO, will replace him.

The change in leadership will not occur immediately and the relevant diplomatic processes are expected to take weeks to complete.

Feinstein was an active member of the public discourse in Poland. Last week, he participated in a ceremony at the Museum of the History of Polish Jews to celebrate the announcement of donations that would allow for the completion of the core exhibition there.

The White House and Secretary of State in Washington did not send any formal explanation as to the reason behind the sudden change, but in diplomatic circles in Warsaw rumors have developed that a falling out between Feinstein and Poland's Foreign Minister Radosław Sikorski could be to blame. According to unofficial sources, the two had a falling out after the American ambassador became too involved in compensating heirs of Jewish assets following the Holocaust.

Feinstein, a graduate of the Georgetown University and former political adviser to American officials, was appointed in 2009 as U.S. Ambassador to Poland by President Barack Obama and was widely supported in a Senate vote. He is considered loyal to U.S. Secretary of State Hilary Clinton. Upon the announcement of his departure, Feinstein wrote that his work in Poland was a great honor and exceptional privilege.