JERUSALEM — More than a decade before the Nazis seized power in Germany, Albert Einstein was on the run and already fearful for his country's future, according to a newly revealed handwritten letter.

His longtime friend and fellow Jew, German Foreign Minister Walther Rathenau, had just been assassinated by right-wing extremists and police had warned the noted physicist that his life could be in danger too.

So Einstein fled Berlin and went into hiding in northern Germany. It was during this hiatus that he penned a handwritten letter to his beloved younger sister, Maja, warning of the dangers of growing nationalism and anti-Semitism years before the Nazis ultimately rose to power, forcing Einstein to flee his native Germany for good.

"Out here, nobody knows where I am, and I'm believed to be missing," he wrote in August 1922. "Here are brewing economically and politically dark times, so I'm happy to be able to get away from everything."

The previously unknown letter, brought forward by an anonymous collector, is set to go on auction next week in Jerusalem with an opening asking price of $12,000.

As the most influential scientist of the 20th century, Einstein's life and writings have been thoroughly researched. The Hebrew University in Jerusalem, of which Einstein was a founder, houses the world's largest collection of Einstein material. Together with the California Institute of Technology it runs the Einstein Papers Project. Individual auctions of his personal letters have brought in substantial sums in recent years.