People attend a memorial service in Joplin, Missouri May 29, 2011. President Barack Obama visited Joplin on Sunday for a memorial for the victims and a first-hand look at the devastation from the tornado, rated an EF-5, or the strongest possible, and the deadliest single twister in the United States since 1947. REUTERS/Eric Thayer (UNITED STATES – Tags: DISASTER ENVIRONMENT)