U.S. military officials involved with President Donald Trump's expanding operations in Latin America have been asked to sign non-disclosure agreements, three U.S. officials say, a development that raises new questions about a military buildup that Venezuela fears may lead to an invasion. The step is highly unusual, given that U.S. military officials are already required to shield national security secrets from public view, and comes as lawmakers in Congress say they are being kept in the dark about key aspects of the mission.