The object was discovered just before 3pm on Monday by sniffer dogs taking part in a joint police and military exercise at the Oskarshamn nuclear plant in south-eastern Sweden.
“Both our and the Armed Forces' dogs reacted to a parcel that was not part of the exercise,” police spokesperson Jimmy Modin told the Aftonbladet tabloid.
“We then took it to the side and x-rayed it. It has not been possible to determine its contents despite this, and for that reason is is being treated as a dangerous object,” he added.
The building where the parcel was found was cordoned off on Monday afternoon while police investigated.
“We have to examine the item and see what it could be before we can say anything else,” said Modin.
Meanwhile, the nuclear plant continued to operate as normal.
Asked shortly after 5pm if police believed there was a threat against the reactors, Modin said: “At the moment we don't think there is. It's located away from the nuclear plant itself.”