Authorities were investigating a threatening letter containing a granular substance that was sent to Donald Trump's son urging an end to his father's presidential campaign, a New York City police official said on Friday.

The letter was addressed to the Manhattan home of Eric Trump, 32, who has appeared frequently on the campaign trail with his father, the Republican front-runner for the White House in 2016.

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"There was a substance inside that is being tested – it's not lethal," the official said.

The letter, which contained threats over Trump continuing his campaign, was being examined by law enforcement experts, the official said. No suspects have been identified.

No injuries were reported.