As the Germans closed in on the city in May 1940, they decided to leave. Although the British offered refuge to French nuclear scientists, including Hans von Halban and Lew Kowarski, they regarded Pontecorvo as an "undesirable". Fortunately, Segrè had been given an offer of employment in Tulsa, Oklahoma, by two European expatriates who were looking for an expert on neutron physics. Segrè had turned down the offer — he already had a good job at the University of California — but recommended Pontecorvo.