An envelope containing suspicious white powder was sent to Trump Tower Thursday, April 27, 2016. Here, the building in Midtown Manhattan is seen on Dec. 8, 2015. Spencer Platt/Getty Images

A suspicious white powder was sent to Trump Tower in New York City, flagship of Republican presidential candidate Donald Trump’s real estate empire, WABC-TV in New York reported Thursday evening.

The powder was discovered around 8:15 p.m. EDT in the fifth-floor mailroom at 725 Fifth Ave. in Manhattan, and police, firefighters and emergency personnel were on the scene investigating, the station reported.

A Trump campaign source told ABC News the fifth-floor offices were evacuated, but most staff had already left for the day. The candidate himself was campaigning in California.

Trump announced his candidacy in the building last summer, and his campaign has an office there.

According to WCBS-TV, an intern opened a letter addressed to Trump and a white powder spilled out. Security in the building resealed the envelope and called 911, and the intern was being isolated and monitored as a precaution, as were a police officer and four other civilians.

Police on Thursday night were testing the substance, but there was no indication that it was dangerous, sources told WCBS.

In March, an envelope containing white power and a threatening letter was sent to the Manhattan apartment of Donald Trump's son, Eric, WNYW-TV noted. The powder was deemed nontoxic but the note threatened the younger Trump that a subsequent envelope would contain hazardous materials if his father didn't drop out of the presidential race, WNBC-TV reported.