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A suspect was detained for questioning and a large area was cordoned off after the object was found around the Gronland subway station in Oslo.
Local witnesses said Oslo's Groenland area, where the device had been found, was cordoned off and security forces removed people from shops and restaurants there.
"It seems like this device only has potential for causing limited damage", the department tweeted.
An Oslo court ruled on Monday that Norwegian police could continue to detain a Russian teenager while they investigate suspicions that he was plotting an attack with a primitive explosive device.
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COPENHAGEN, Denmark (AP) - A 17-year-old asylum-seeker from Russian Federation was arrested Sunday in connection with an explosive device found near a busy subway station in Norway's capital that police defused before it detonated, authorities said.
Norway's security service PST said early Sunday it had taken over the investigation for several reasons but did not elaborate.
Officers evacuated late-night bars and restaurants in the Norwegian capital as bomb disposal experts rushed to the scene. "The police were screaming at us to get back inside and stay where we were", she added by telephone. A 39-year-old native of Uzbekistan, accused of carrying out that attack, has been arrested.
In 2011, right-wing extremist Anders Behring Breivik set off a auto bomb in Oslo that killed eight people and destroyed Norway's government headquarters, before going on a shooting rampage, killing 69 people at nearby Utoeya island. An additional 69 people were killed afterwards when he went on a shooting spree at nearby Utoeya island.