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COPENHAGEN, Denmark (AP) — Four Russian nationals — three men and one woman — have been arrested and held in custody after they were seen takign pictures of objects that are subject to a ban on photography in central Norway, police said Monday.

Officers found “photographic equipment and relatively extensive image material” in their possession when their car was stopped on Oct. 11. Police didn’t say what they were photographing. The four — none of whom were identified — came to Norway from Finland, police said, adding that they said there were tourists.

Authorities said they was no immediate link to two other cases of Russians detained elsewhere in Norway after police found them in possession of drones. Numerous drone sightings have been reported near Norwegian offshore oil and gas platforms in recent weeks.

Police declined to give further details “due to the nature of the case.”

There is heightened security around key energy, internet and power infrastructure following last month’s underwater explosions that ruptured two natural gas pipelines in the Baltic Sea that were built to deliver Russian gas to Germany.

The blasts happened off both Sweden and Denmark, in international waters but within the countries’ exclusive economic zone. The damaged Nord Stream pipelines discharged huge amounts of methane, a potent greenhouse gas, into the air.