Alvarsjo said Swedish police have "unfortunately" noticed a rise in the use of hand grenades by criminal groups in the Nordic country.
The man, in his 60s, was rushed to a hospital but later died, while the woman, in her mid-40s, suffered minor wounds to her face and both legs, Olsson said.
Police activity near Varby Gard metro station
The underground station of Varby Gard s on the Red Line (Roda linjen) on the way to Norsberg in Stockholm's south-west and an adjacent square was closed while police carry out checks, they said in a statement.
According to local media, a 60-year-old man is critical condition, because the device exploded in his hands as he lifted it from the ground. "I don't dare imagine what would have happened if one or several children had found the hand grenade".
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Police checked in and around the subway station after the blast to make sure there were no other explosive devices.
Sven-Erik Olsson of the Stockholm police told reporters that he could not say what kind of explosive material was used or how powerful the explosion was, only that it was "powerful enough for the couple to be taken to hospital".
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Police have cordoned off an area outside the entrance to the metro station Varby Gard, and are investigating the incident.
Officials say the blast is not believed to be terror-related.
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