Two people were injured when a parcel bomb exploded in the offices of the Swiss nuclear lobby, police said on Thursday.
The two female employees of Swissnuclear were taken to hospital with superficial burns and hearing damage, a police spokesman said, adding the mail had exploded on Thursday morning but police did not yet know who sent it.
Police cordoned off the office of Swissnuclear in the northern Swiss town of Olten and the spokesman said they had forensic specialists on the ground.
Earlier this month, Switzerland suspended the approvals process for three new nuclear power stations so safety standards could be reviewed, after Japan's earthquake and tsunami sparked a disaster at the Fukushima nuclear power plant.
Switzerland derived about 10 percent of its energy from nuclear generation in 2009, according to the Swiss Federal Office of Energy.