In January 1944 her father sent her to observe the opening of a mass-grave of 11,000 Polish soldiers. Their deaths was seen as a war crime. Both Germany and the Soviet Union invaded Poland under the terms of Molotov-Ribbentrop Pact, and, in 1954, Mortimer was called as a witness to try to determine which nation had performed the mass summary execution. Mortimer's conclusion was that the Germans were responsible for the killing, and that the limited evidence that Soviets had been responsible was a German ploy. That was later proven incorrect, and it has been widely established that the crime was carried out by the Soviets.