Emergency crews responded to the Subway Headquarters in Milford Thursday afternoon for a report of a letter with white powder inside.
Fire officials said one person was directly exposed and two others nearby when the letter was opened.
The Milford Fire Department's hazmat team, along with Connecticut State Police and members of the FBI responded and determined the substance was not hazardous.
No one was hurt.
The situation is being treated as a criminal matter and the investigation is ongoing.