Staff were put on lockdown after white powder and a threatening note were sent to a credit card call centre in Chatham.
The suspicious package, which was posted to the Vanquis Bank contact centre in Chatham Maritime, was opened by mail room staff who immediately called the emergency services.
The note reportedly said staff should "die slowly".
Around 700 employees at the call centre, which is in Pembroke Court on the Universities of Medway campus, were put on lockdown and had to stay at their desks until the matter was dealt with.
However, after police and firefighters were sent to the scene, it was quickly established the powder was not harmful.
One woman, who didn’t want to be named, said: “My friend works there and told me what happened this morning.
“Apparently all the white powder fell out when it was opened and the letter said 'die slowly', how frightening that must have been for the person who opened it.
“My mate told me after the emergency services had been that the powder was harmless and turned out to be wheat flour or something similar."
A spokesman for Kent Fire and Rescue Service, said: "We were called at 12.21pm to Pembroke Court after a white substance was found.
"We used our detecting equipment to establish the substance was non-harmful and it turned out to be wheat flour.
"We handed the incident back to the police."
A spokesman for police said: “At 10.49am on Monday 13 March 2017 Kent Police received a report stating that a suspicious package had been sent to a premises in Pembroke Court, Chatham.
“The matter was referred to Kent Fire and Rescue Service, who attended the scene. Officers also attended the scene to provide support.
“The package was not found to be harmful and there are no reports of anyone sustaining injuries.
“Inquiries into the circumstances are ongoing.”