Advocacy group Greenpeace said Russian investigators have closed the books on its probe into the actions of activists dubbed the Arctic 30.

Greenpeace last year used its Arctic Sunrise vessel to gain access to the Prirazlomnaya rig, deployed by Russian energy company Gazprom for work in the country's arctic waters. Two freelance journalists and 28 Greenpeace activists, dubbed the Arctic 30, were held by Russian authorities on piracy charges last year.

Greenpeace said its lawyers were informed by the Kremlin that the investigation was officially closed.

"Since this story began, the [Russian] Investigative Committee has tried to bend the rule of law to persecute those who dare to oppose Arctic oil drilling," Greenpeace International Executive Director Kumi Naidoo said in a Wednesday statement. "This persecution has had the opposite effect, and our movement is much stronger both in Russia and around the world."

The Russian Foreign Ministry said the action "had the appearance of extremist activity." Greenpeace said it was concerned about the potential for an oil spill in the harsh arctic environment.

Greenpeace activists in May staged a similar protest against Gazprom and Norwegian energy company Statoil for their planned operations in the frigid northern climate.

In New Zealand, activists smeared themselves Thursday with fake oil to protest against Statoil's campaign there.