Investigations are continuing to identify a substance which was discovered in an envelope, delivered to the Alliance and Leicester offices in Prospect Hill, Douglas this morning.
CID officers are working to trace the origins of the envelope and the white powder it contained.
The Department of Home Affairs says the response phase of today's incident has ended and attention will now focus on a potential criminal investigation.
This morning's discovery prompted a full turnout of the Island's emergency services, a precautionary evacuation of the building and temporary closure of a section of Prospect Hill.
The road reopened after about two hours.
Specialist officers from Merseyside have arrived on the Island to analyse the powder.