The precipitous fall of Hernández, who was president from 2014 until January, has stunned citizens in this Central American country of 10 million. Less than three years ago, President Donald Trump praised the conservative leader for “working with the United States very closely” to curb migration. But at the same time, federal authorities were preparing to prosecute his brother as a drug kingpin. That trial, which ended in a conviction, included references to the president’s alleged criminal conduct.