Police testing 'granular substance' in threatening letter to Trump's son

US authorities are attempting to trace the sender of a threatening letter, containing what was described as “a granular substance,” sent to presidential candidate Donald Trump's son, urging him to stop his father's presidential campaign, a New York City police official said on Friday.

Police had been called to Trump Parc East, a luxury apartment building in the Manhattan are of New York on Thursday evening after receiving a report of a suspicious letter received by a tenant.

The letter had been addressed to 32 year-old Eric Trump, who has made frequent appearances on the campaign trail with his father, the Republican front-runner for the White House in 2016.

The letter, which contained threats over Trump continuing his campaign, was being examined by law enforcement experts, the official said.

The substance inside the envelope was being tested but was “not lethal," the official said.

No suspects have been identified and no injuries are reported.