Police in Frankfurt said that employees in the ECB's post department sounded the alarm today morning when they saw black powder seeping out of the letter.
One of the employees complained that she was feeling unwell after handling it and police and the fire brigade cordoned off the area as a precaution.
However, following an initial examination, no suspicious chemicals were detected. And the employee concerned said the symptoms had since disappeared.
As a result, the employees were allowed to return to their working places a short while later.
There was no information regarding either the sender or the addressee of the letter.