"At about 8.15 am, while opening the letter, it exploded, injuring two people," a spokesman for police in the canton of Solothurn told AFP.
He said the blast occurred at the offices of Swissnuclear, the nuclear energy offshoot of the Swiss power industry association.
It groups major power generating firms in Switzerland, according to the association's website.
The spokesman did not give the identity of the victims or details on the gravity of their injuries.
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