President Barack Obama's handshake with Cuba's President Raul Castro Tuesday at a memorial for the late South African leader Nelson Mandela was not "pre-planned," a US official said. "This wasn't a pre-planned encounter," a White House official said. "Above all else, today is about honoring Nelson Mandela, and that was the president's singular focus at the memorial service. We appreciate that people from all over the world are participating in this ceremony."