Hall was scheduled to return to Los Alamos a few days later so Kurnakov was pressed to make a determination whether Sax and Hall were legitimate in their offer or were undercover agents for the FBI. The decision was made that two potential for gaining valuable information far exceeded the accompanying risks.

Moscow received a cable on November 12, 1944, detailing the offer by the young spies. Hall was given the code name "Mlas" which meant young and Sax, a year older than Hall was referred to as "Star" meaning old. Hall obtained and Sax delivered numerous documents and design pals for atomic weapons research including information about implosion experiments to the Soviets.

The United States learned of Hall's espionage activity when it deciphered intercept Soviet cable messages. These messages, known as the Venona documents provided clear evidence of Hall and Sax. However, the United States, unwilling to alert the Soviets to the fact that they had broken the Soviets code, confronted Hall but did not pursue legal action against him. Hall went on to become a noted biophysicist at Cambridge University, working with biological X-ray research.

In 1996, the Venona documents were made public by the NSA. Hall, in ill health with cancer and Parkinson's disease, acknowledged that he may have been wrong about the Soviet government, but refused to apologize for his actions. Hall died in 1999.