World leaders gathered here for a summit on nuclear security observed a moment of silence Tuesday in memory of the Polish president and other officials killed in a plane crash in Russia. Polish President Lech Kaczynski perished along with his wife, senior Polish military leaders and politicians in Saturday's plane crash near the western Russian city of Smolensk. The crash claimed the lives of 96 people. They were on a visit to commemorate the massacre of Polish officers by the Soviets during World War II.