Story highlights Donald Trump's sister received a threatening letter, a law enforcement official told CNN on Monday

Trump's son received one last week

(CNN) Donald Trump's sister received a threatening letter similar to one recently sent to the Republican presidential front-runner's son, a law enforcement official briefed on the investigation told CNN Monday.

The letter sent to federal appeals court judge Maryanne Trump Barry threatens harm if her brother doesn't drop out of the presidential race, the law enforcement official said.

Trump is pictured with his sister Maryanne Trump Barry as they adjourn for lunch during a public inquiry over his plans to build a golf resort on June 10, 2008 in Aberdeen, Scotland.

Eric Trump received a similar letter, though his contained a suspicious powder that was tested and appeared to be lemonade mix. Law enforcement is investigating whether both letters were sent by the same person.

The investigation is being coordinated through the FBI's Joint Terrorism Task Force because of the white powder that was sent to Eric Trump, the law enforcement official said.

Investigators anticipate more letters and have advised Trump family members to put extra precautions in place to screen their mail, the official said.

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