Several envelopes were posted on Wednesday to the offices of Italy's debt collection agency Equitalia in several other northern Italian cities.
Fire service and police nuclear, biological and chemical weapons experts sealed off Equitalia's offices in Turin, Biella and Aosta after the enveloped arrived.
Two Equitalia employees in Aosta who handled the envelopes and eight others who were in close proximity to them were taken to hospital for medical tests but none showed immediate signs of poisoning.
Investigators are working to identify the white powder, which was said to contain organic material and who may have sent it to Equitalia.
The highly unpopular agency was targeted by several anarchist letter bombs in 2012 and is being abolished under Italy's 2017 budget.