The FBI has obtained a warrant to begin reviewing newly discovered emails that may be relevant to the Hillary Clinton email server investigation. Less than two weeks before election day, the timing of the FBI Director James Comey's letter announcing this decision has drawn criticism from many.
Democrats and some Republicans have said that the decision potentially tips the scales in the US presidential race in favour of Donald Trump.
Dozens of former federal prosecutors, including former Attorney General Eric Holder, have signed a letter critical of Comey's decision.
And Senate Minority Leader Harry Reid wrote to Comey saying the action may have violated a law that generally restricts the political activities of federal employees.
He also suggests in his letter that the FBI have "explosive information" about ties between Trump and the Russian government that it is withholding from the public.
Trump has been energised by the news, calling the latest developments worse than Watergate and arguing that his candidacy has the momentum in the final days of the race.
He also highlighted reports that the Justice Department had discouraged the FBI from alerting Congress to the unexpected discovery of the emails, and said the department is trying "so hard" to protect Clinton.