Officers were first called Wednesday to the court building in Thompson, where two court workers who handled and opened a large envelope experienced breathing problems.
Their eyes and throats also started burning, said RCMP Assistant Commissioner Kevin Brosseau.
The two were treated at the scene and did not need to go to hospital.
Brosseau said the building was evacuated and an alert went out to other courthouses in the province.
Similar envelopes were then found at courts in Winnipeg, Brandon, Morden and Minnedosa. They had not been not opened.
Tests are being done on all the packages, Brosseau said, but an initial examination suggests they do not contain a powder or anything toxic.
"This isn't anything of a significant noxious type," he said, adding a proper analysis could take some time.
He said it's believed the public is not at risk.