US Republican presidential front-runner Donald Trump’s sister has also received a threatening letter, just one day after a similar letter containing white powder was sent to the billionaire’s son.

Maryanne Trump Barry, a federal circuit judge, received the letter in Philadelphia on Friday.

The Federal Bureau of Investigation (FBI) and Secret Service, which is in charge of ensuring the safety of important politicians, are investigating the letter, according to NBC.

Barry’s letter didn’t contain any powder, but the threats in the letter were similar to the one sent to Eric Trump on Thursday. Eric’s letter demanded that his father drop out from the Republican race or face the consequences.

A source close to the investigation told CBS News that Eric’s letter had a Massachusetts postmark and warned that if the New York businessman doesn't withdraw from the Republican race, then the children will be harmed.

The handwritten note said, “If your father does not drop out of the race, the next envelope won't be a fake.” It was signed “X.”

Trump, who has never held elected office, is leading the Republican race despite the fact that his campaign has been marked by controversial statements.

Trump ruined Bush's candidacy

Donald Trump (left) talks to Jeb Bush during a Republican presidential candidate.

Trump systematically destroyed the candidacy of Jeb Bush, who was once the presumptive Republican Party nominee. Bush, brother and son to two former US presidents, dropped out last month.

He had repeatedly attacked Bush throughout his campaign for the Republican nomination, calling him "weak," "low-energy" and "an embarrassment" to the Bush family.

Trump also targeted Jeb’s brother, George W. Bush, over Iraq war and the September 11, 2001 terrorist attacks.

In a heated exchange between Jeb and Trump during the ninth Republican presidential debate on February 13, the New York billionaire blamed George W. Bush for the collapse of the World Trade Center and the death of 3,000 Americans.

“The World Trade Center came down during your brother’s reign, remember that,” Trump said.

The September 11 attacks, also known as the 9/11 attacks, were a series of strikes in the US which killed nearly 3,000 people and caused about $10 billion worth of property and infrastructure damage.

US officials assert that the attacks were carried out by al-Qaeda terrorists but many experts have raised questions about the official account. They believe that rogue elements within the US government orchestrated or at least encouraged the 9/11 attacks in order to accelerate the US war machine and advance the Zionist agenda.

'Trump could be killed for 9/11 truth'

According to American scholar Dr. Kevin Barrett, the full truth about the 9/11 Zionist coup d’état is probably too politically explosive for Trump to ever tell since it would destabilize the US political system.

Dr. Barrett, a founding member of the Scientific Panel for the Investigation of 9/11, told Press TV on January 13 that if the Republican presidential frontrunner did that he would be immediately taken out physically or attacked quite brutally in the media.

“Donald Trump is getting a little bits and pieces of truth, but the full truth probably is too politically explosive for Donald Trump to ever tell—not that it would help him politically to tell it, because he would be immediately shut down and taken out—whether physically, by being killed, which certainly could happen, or by being attacked quite brutally in the media,” said Dr. Barrett, the author of Questioning the War on Terror.