Mrs. Clinton’s use of a private email server while she was secretary of state emerged early as a potentially serious problem for her campaign. However, she initially made jokes about the issue, responding dismissively to questions about it. Her failure to quickly put the matter to rest was a source of frustration for Democratic leaders early on. In the final weeks of the race, her campaign was rocked when law enforcement officials said emails pertinent to the closed investigation were discovered on a computer belonging to Anthony Weiner, a former congressman from New York and the estranged husband of Huma Abedin, a top aide for Mrs. Clinton.