MOSCOW — Russia charged five Greenpeace activists with piracy over the boarding of an oil platform to protest Arctic drilling in what the group described as its worst crisis since the 1985 bombing of the Rainbow Warrior.

The five, who include citizens of Britain, Brazil, Finland and Russia, as well a Swedish-U.S. dual national, are among 30 Greenpeace activists who were detained last month over the incident, the organization's press service said in a text message Wednesday. They're under investigation for piracy, a charge that carries a maximum prison sentence of 15 years.

"This is now the most serious threat to Greenpeace's peaceful environmental activism since agents of the French secret service bombed the Rainbow Warrior" during an attempted protest over France's nuclear weapons testing in the Pacific, the organization's executive director, Kumi Naidoo, said by e- mail. The 30 activists "could now face the prospect of long periods in a Russian jail."

Investigators opened a criminal case last week against the campaigners from 18 countries for piracy and remanded them in custody for two months in the port city of Murmansk. The Investigative Committee will make an announcement once charges are filed against all 30 activists, according to the office of the agency's spokesman, Vladimir Markin.

Greenpeace protesters scaled an OAO Gazprom rig in the Pechora Sea on Sept. 18. The state-owned company has accused the environmental organization of endangering the lives of workers on the rig who were underwater at the time of the protest.

Russia's Coast Guard boarded Greenpeace's Arctic Sunrise ship in international waters on Sept. 19, a day after two protesters scaled the rig, and towed the vessel to Murmansk. Gazprom plans to become the first Russian company to start producing oil in Arctic waters at the Prirazlomnoye deposit as soon as this year. Greenpeace activists scaled the same drilling platform in 2012.

The detained activists include citizens of the U.S., Finland, Argentina, Switzerland, Britain, Australia, Brazil, Canada, Denmark, the Netherlands, New Zealand, Ukraine, Russia, France, Italy, Turkey, Poland and Sweden, according to Greenpeace.

Russian President Vladimir Putin said Sept. 25 that while the Greenpeace campaigners "clearly" aren't pirates, they violated international law by trying to seize a drilling platform.

The Russian leader has come under increasing criticism in Europe and the U.S for a crackdown on civil society since he won a third Kremlin term last year.

The captain of the Arctic Sunrise, American Peter Wilcox, was in charge of Greenpeace's Rainbow Warrior in 1985, when agents from France's DGSE intelligence service planted a bomb on the ship in the port of Auckland to prevent it from sailing toward a nuclear-testing site. A photographer for the environmental organization died in the explosion that sank the vessel.

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