U.S. authorities are investigating a threatening letter sent to Republican presidential candidate Donald Trump's sister, a federal appeals court judge in Philadelphia, according to NBC News.

Maryanne Trump Barry, 78, received the letter on Friday, the network's Philadelphia affiliate said in a report late on Sunday, citing a source familiar with the investigation.

Reuters could not immediately confirm the report.

Separately on Friday, officials said they were investigating a letter sent to Trump's son Eric that contained a granular substance.

The FBI and the U.S. Secret Service are investigating the letter to Trump's sister. The source told NBC that letter did not contain any white powder but did have the same threatening message that demands Trump drop his bid to be the Republican presidential nominee.

Trump is leading among the three remaining Republicans campaigning to represent the party in the Nov. 8 presidential election.