"We had information that suggested the need for a review of security measures," said Eirik Bergesen, a spokesman for the foreign ministry in Oslo. "We have done that, and believe the conditions allowed it to reopen."

On Monday, the Latvian Interior Ministry blamed an Islamic extremist group based in a Nordic country for the threat, but refused to identify the group or which country it was based in.

The Norwegian closing coincided with warnings from US embassies in Finland and Latvia urging Americans to be wary about going to shopping centers and using public transport in the Nordic and Baltic Region.