Updated

Greenpeace said Wednesday that five of its activists who were detained after protesting at a Russian oil platform have been charged with piracy and could face up to 15 years in prison if found guilty.

The environmental group said in a statement that activists from Brazil, Britain, Finland and Sweden as well as a British videographer have been formally charged with piracy.

The Russian Coast Guard seized a Greenpeace ship and all 30 people from 18 countries on board following the Sept. 18 protest at an offshore platform in the Arctic owned by Russian state-controlled energy giant Gazprom.

Greenpeace said in a statement that more activists are expected to be formally charged on Thursday.

The investigators said the Greenpeace ship, the Arctic Sunrise, had violated the 500-yard security zone around the platform and that it was carrying equipment whose purpose was still unclear.

Greenpeace has said its ship stayed out of this zone but its inflatable boats used by activists to reach the platform and then scale it posed no danger.

The activists have been in custody in the northern city of Murmansk since last week.