Melanie Eversley

USA TODAY

New York City police are investigating an envelope filled with white powder that was found in a Trump Tower mailroom Thursday night, the agency said.

Police were called to the Manhattan complex after a worker opened an envelope mailed to a Donald Trump campaign office, the Associated Press and NBC News reported.

The substance was tested and deemed not hazardous a few hours later, but authorities said it would they would need to do additional testing to determine what it is, according to the AP.

Six people, including a police officer, were evaluated by medical crews as a precaution, ABC News reported. The fifth-floor offices were evacuated though few people were there at the time.

Reports: Police investigate threatening note sent to Trump's son

In March, someone sent white powder to the GOP presidential front-runner's son, Donald Trump Jr., prompting police to investigate. That envelope was deemed not hazardous, but it came with a letter that read, "If your father does not drop out of the race, the next envelope won't be a fake," AP reported.

Donald Trump's older sister, federal appeals court judge Maryanne Trump Barry, also received a threatening letter about the presidential candidate.

In the month since those threats, Trump has solidified his lead in the Republican primary race. He swept in all five of Tuesday's primaries in the Northeast, finishing the night with a total of 994 delegates over Cruz's 566 and Kasich's 153.

Donald Trump's sister, like son, receives threatening letter