Story highlights 30 were detained after a protest at a Gazprom oil platform in the Barents Sea

Greenpeace says two activists tried to hang a banner from a rig

Russia: The action endangered lives, and could have led to an environmental disaster

The activists are "accused of an imaginary offense," a Greenpeace official says

Russian authorities have charged all 30 people aboard a Greenpeace ship with piracy after two activists tried to scale an oil platform in a protest of Arctic drilling last month, the Russian Investigative Committee said Thursday.

The charging began Wednesday and finished Thursday. If convicted, the 28 activists and two freelance journalists could be sentenced to up to 15 years in prison.

None of the 30, who hail from at least 18 countries, pleaded guilty, the committee said.

The defendants include Americans Peter Wilcox, who is the captain of the Arctic Sunrise, and Dmitry Litvinov, who Greenpeace says also holds Swedish citizenship.

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They were arrested after two of the activists left the Arctic Sunrise and tried to climb the side of an oil platform owned by the Russian energy giant Gazprom in the Barents Sea on September 18.

The Russian coast guard detained the pair and the rest of the Arctic Sunrise's crew, and towed the ship to the northwestern Russian port city of Murmansk.

Greenpeace said the two activists were trying to hang a banner from the side of the rig in what the group called a peaceful protest against the "slow but unrelenting destruction of the Arctic."

But Russian authorities accuse them of trying to commandeer the platform. The authorities and Gazprom also say the activists endangered the lives of the company's employees and that their action could have led to an environmental disaster.

Lawyers acting on Greenpeace's behalf have filed appeals in court seeking the defendants' release, Greenpeace said Thursday.

"Our activists have been charged with a crime that did not happen. They are accused of an imaginary offense," Greenpeace International Executive Director Kumi Naidoo said Thursday. "There can be no doubt about why the charge of piracy has been brought and the legal hammer wielded.

"An effort is under way to intimidate us, but our peaceful, passionate campaign against Gazprom and all other Arctic drillers will not be silenced."

The charge of piracy is "over the top," Phil Radford, executive director of Greenpeace USA, told CNN earlier this week.

"I think they (Russian prosecutors) will take it all the way to trial," Radford said. "I think Gazprom is using its political muscle to have the courts really crack down on these peaceful protesters. It's a serious and overblown charge.

"They need to have the piracy charge because it's the only way they can save face from illegally arresting these activists and journalists."

Russian President Vladimir Putin has indicated he does not consider the protesters pirates, but Prime Minister Dmitry Medvedev, quoted in the Russian media, said Wednesday: "Environmental concerns cannot be used as a cloak for illegal acts no matter what lofty considerations such acts are based on. They cannot manifest themselves in illegal methods or methods that are eventually unsafe for people and technological facilities."