Protest vessel Arctic Sunrise, seized last year by Russian authorities, is back in the hands of Greenpeace, the advocacy group said Friday.

"Our lawyers have signed the papers and she's officially back in our hands," Greenpeace International's top arctic campaigner, Ben Ayliffe, said in a statement.

The ship was seized following the September arrest of 28 Greenpeace activists and two freelance journalists, dubbed the Arctic 30. Last year, the group used the vessel to gain access to the Prirazlomnaya rig, deployed by Russian energy company Gazprom for work in the country's arctic waters.

Ayliffe said the Arctic Sunrise was "illegally seized at gunpoint" by Russian authorities while it was positioned in international waters.

"The Arctic Sunrise should never have been detained in the first place," he said.

The Russian Foreign Ministry said the actions taken by Greenpeace activists "had the appearance of extremist activity capable of causing human deaths and other grave consequences."

Greenpeace said it was concerned about the potential for an oil spill in the harsh arctic environment. Activists in May staged a protest against Gazprom and Norwegian energy company Statoil for their planned operations in the frigid northern climate.