Greg Toppo

USATODAY

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, authorities said.

The letter sent to Trump Barry contained no white powder, NBC News reported, but the threat was similar to one sent to Eric Trump, a source familiar with the investigation said.

The FBI and Secret Service were investigating the incident.

Eric Trump's letter, received Thursday, demanded that his father drop out of the Republican race for the presidential nomination. It wasn't immediately clear if both letters were sent from the same person. The letter sent to Eric Trump was postmarked in Massachusetts, police sources told NBC, but preliminary tests found that the substance was not hazardous.

"The FBI is aware of the incident and is working closely with the United States Secret Service and U.S. Marshals Service," the FBI Philadelphia said in a statement. It didn't offer further details.

Donald Trump's representatives didn't immediately respond to a request for comment, NBC reported.

Trump Barry, a 78-year-old judge with the U.S. Circuit Court of Appeals for the 3rd Circuit, was a subject of scrutiny earlier this year when Trump mentioned her in jest when asked about whom he would appoint to the U.S. Supreme Court. GOP rival Ted Cruz called her a "radical pro-abortion extremist" for her ruling against banning abortions in New Jersey.