The FBI and Secret Service are investigating a threatening letter sent to Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE's sister, according to NBC.

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Maryanne Trump Barry, a federal circuit judge, received the letter in Philadelphia Friday, just one day after a similar letter containing white powder was sent to Trump’s son Eric.

NBC reports that while Barry’s letter didn’t contain any powder, the threats in the letters were similar.

Trump’s letter demanded that his father, the front-runner in the race for the GOP nomination, drop out.