'Anthrax' scare at Northampton Royal Mail office
A letter with "anthrax" written on it burst open in a Royal Mail sorting office covering workers with white powder.
Two staff members at Northampton's Swan Valley office had decontamination treatment as a result.
A union representative said a message on the envelope read: "You were just killed with anthrax."
Eleven staff who had been in contact with the powder, believed to be flour, were treated at the scene.
A police spokesperson said: "An area of the sorting office was evacuated as a precaution."
An East Midlands Ambulance Service spokeswoman, said: "Several of our ambulance clinicians were dispatched to the Royal Mail premises, including our hazardous area response team.
"Eleven patients were assessed at the scene by our crews, two of whom were decontaminated by the fire service because they'd come into direct contact with the substance which was being tested at the time."
Gareth Eales, from the Communication Workers Union (CWU), said: "It was possibly hate mail, but the staff did the right thing by reporting it to the authorities."
The powder was found to be "innocuous" and emergency crews left the building at about 02:00 BST, four hours after arriving.