Threatening letter sent to Donald Trump's sister prompts investigation: Source

Donald Trump's older sister, federal appeals court judge Maryanne Trump Barry, received a threatening letter in Philadelphia on Friday — one day after the GOP front-runner's son received a suspicious piece of mail containing white powder, sources told NBC News.

Related:N.Y. Police Investigating Suspicious Powder Sent to Donald Trump's Son Eric

There was no white powder sent to Trump Barry, although the threat included in her letter was similar to that given to Eric Trump, a source familiar with the investigation said. Eric Trump's letter, received Thursday, demanded that his real estate mogul father drop out of the Republican race for the presidential nomination.

It's unclear if both letters have the same sender. The FBI and Secret Service are investigating the incident.

See Trump on the campaign trail:

Trump Barry, a 78-year-old judge with the U.S. Circuit Court of Appeals for the 3rd Circuit, was the subject of some scrutiny earlier this year when Trump mentioned her in jest when asked about whom he would appoint to the Supreme Court.

Rival Ted Cruz, meanwhile, called Trump Barry a "radical pro-abortion extremist" for her ruling against banning abortions in New Jersey.

The mail sent to Eric Trump was postmarked in Massachusetts, police sources said, but preliminary tests deemed the substance non-hazardous.

Calls for suspicious and threatening packages are common for high profile personalities.

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