Trump Tower
A New York Police Emergency Service truck outside the Trump Tower during an investigation into a suspicious white powder substance found inside the tower.Reuters

New York City authorities are investigating after a suspicious white powder was discovered at Trump Tower near Donald Trump's campaign office on 28 April. Law enforcement responded to the location around 8.05pm EDT/1.05am BST when the powder was found on the fifth floor of the Midtown Manhattan building.

According to New York Fire Department spokesman John Ryan, five staff members and a police officer were isolated by emergency personnel and evaluated following the incident. Police said a Trump staff intern opened the letter containing the substance and immediately called authorities, The New York Times reported.

CBS News reported security resealed the envelope and called 911. The Secret Service, which is tasked with protecting the Republican frontrunner, was also notified. It was unclear if the envelope containing the substance also contained a letter.

Sources told CBS New York that police were testing the suspicious powder, but there were no signs that it posed a threat. While Trump's campaign directed reporters to the Secret Service for comment, a campaign source told ABC News that the fifth floor was evacuated but that most of the staff had already left for the day.

During a previous incident in March, an envelope containing a non-hazardous white powder and threatening message from Massachusetts was sent to the Manhattan flat of Trump's son Eric. A day later, the FBI and Secret Service investigated a threatening letter sent to Trump's sister Maryanne Trump Barry, a federal appeals court judge. The investigation is ongoing.