"Republicans are doing so well in early voting, and at the polls, and now this 'Bomb' stuff happens and the momentum greatly slows — news not talking politics. Very unfortunate, what is going on. Republicans, go out and vote!" he tweeted.

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The president's use of scare quotes around the word "bomb" appeared to reflect his skepticism about the story, which has dominated cable news for days.

Some of Trump's allies have suggested, without evidence, that the packages are part of a "false flag" operation by liberals to hurt Republicans ahead of next month's midterm elections.

Minutes after Trump's tweet, the Justice Department announced one person is under arrest in connection with the plot.