New York City firefighters and police said on Thursday they were investigating a report that a suspicious white powder substance was found inside Trump Tower in Midtown Manhattan.

New York Police spokesman Brendan Ryan said the powder was found in an office on the fifth floor of the building, which houses the campaign headquarters of Donald Trump, the Republican presidential front-runner. No injuries were reported and an investigation was underway, he said.

Five civilians and a police officer were isolated by emergency personnel and were under evaluation after authorities responded to the location around 8:05 p.m. EDT (0005 GMT), New York Fire Department spokesman John Ryan said.

Trump campaign spokeswoman Hope Hicks directed all questions to the US Secret Service. A representative for the agency could not be reached immediately for comment.

Last month, a threatening letter containing a granular substance was sent to Trump’s son, Eric, urging an end to his father’s presidential campaign.