Two people have been injured when a letter bomb exploded at an office of the Swiss nuclear power industry in the northern city of Olten.
The explosion happened as staff were opening the morning's post in the fourth-floor office of Swissnuclear, a lobby group representing several Swiss power companies.
Solothurn cantonal (state) police spokeswoman Thalia Schweizer said two female staff members had been taken to a hospital with superficial injuries.
The motive for the attack is still unclear.
"We can't speculate, but we are investigating," Schweizer said, adding that no claim of responsibility has been received.
Opponents of nuclear power in Switzerland have become more vocal in recent weeks as images of the stricken Fukushima reactor in Japan appear on the evening news bulletins daily.
Earlier this month the Swiss government froze plans for the construction of new nuclear plants pending a safety review prompted by the radiation leak in Japan.
Switzerland has five nuclear reactors supplying some 40 % of the country's electricity.