MOSCOW — Environmental activists who conducted a protest at a Russian Arctic offshore oil platform last week will be prosecuted and could face piracy charges that are punishable by up to 15 years in prison, Russian investigators said on Tuesday.

It said the "attack" in which two Greenpeace activists tried scaling Gazprom's Prirazlomnaya platform, Russia's first offshore Arctic oil platform, had threatened environmental damage to the region and violated Russian sovereignty.

"When a foreign ship full of electronic equipment intended for unknown purposes and a group of people, declaring themselves to be environmental activists, try to storm a drilling platform there are legitimate doubts about their intentions," said the statement.

The protest ended in the arrest of the nearly 30 activists aboard the ship and the detention of the Netherlands-registered vessel. Russian authorities were towing the Arctic Sunrise toward the Arctic port of Murmansk on Tuesday.

Greenpeace has said Russia's actions violated international law and has demanded the immediate release of the activists. The group said protesters aboard the ship had been denied access to lawyers and consular officials for four days.