A white powder that was discovered at the campaign office of Donald J. Trump at the Trump Tower in Midtown Manhattan on Thursday night was not a threat, the authorities said.

The Police Department said it was called around 7:30 p.m. to the fifth floor of the tower, at Fifth Avenue and 56th Street, for a report of an envelope containing white powder. Mr. Trump’s home is also in the building.

The envelope had been mailed to Mr. Trump’s campaign office, where a staff member opened it and immediately called the police, The Associated Press reported.

The police said early on Friday morning that the powder was deemed to be safe, though investigators were still determining what exactly it was. The situation was declared under control around 11:10 p.m., the Fire Department said.