Russia on Tuesday filed piracy charges against Greenpeace activists who tried to climb onto an offshore drilling platform in the Arctic owned by the state-controlled gas company Gazprom.

The activists are on a Greenpeace ship, the Arctic Sunrise, which was seized last week by the Russian Coast Guard and towed on Tuesday into a port near Murmansk.

It was unclear how many of the 30 activists on board face piracy charges, which carry a potential prison sentence of up to 15 years and a fine of 500,000 rubles (about $15,500). The Investigative Committee, Russia’s federal investigative agency, said it would question all those who participated in the protest and detain the “more active” among them.

Two activists tried to climb onto the Prirazlomnaya platform on Thursday and others assisted from small inflatable boats. The Greenpeace protest was aimed at calling attention to the environmental risks of drilling for oil in Arctic waters.

“When a foreign vessel full of electronic technical equipment of unknown purpose and a group of people calling themselves members of an environmental rights organization try nothing less than to take a drilling platform by storm, logical doubts arise about their intentions,” Investigative Committee spokesman Vladimir Markin said in a statement.

Greenpeace insists that Russia had no right under international law to board its ship. One activist told The Associated Press that the Coast Guard officers hit and kicked some activists when they stormed the vessel.

The Arctic Sunrise was anchored on Tuesday in a small bay near Severomorsk, the home port of Russia’s Northern Fleet, 25 kilometers (15 miles) north of Murmansk.

Greenpeace said the 30 activists were from 18 countries.

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