MOSCOW -- Russian officials on Wednesday dropped piracy charges against Greenpeace activists who were jailed last month after protesting oil drilling in the Arctic, charging them instead with hooliganism.

An investigation led officials to issue the less severe charges of hooliganism, which carry a maximum penalty of seven years, instead of piracy, which could mean up to 15 years in prison, Russia's Investigative Committee said in a statement.

“A big volume of work was conducted by the investigators, which established an objective picture of the events that happened,” investigative committee spokesman Vladimir Markin said late Wednesday on the agency's website.

All the detainees were denied bail and remained in custody pending further investigation, officials said.

Markin said in the statement that the detainees' refusal to cooperate with the investigation prevented the facts from being established sooner. Some activists may face additional charges with penalties of up to 10 years in prison, he said.

Greenpeace hailed Russia's decision to issue the lesser charges as the first step toward eventually freeing the activists.

“The decision really took us by surprise, which goes to show that Russian authorities' actions are often as illogical as unpredictable,” Vladimir Chuprov, head of Greenpeace Arctic program, said in an interview with the Los Angeles Times.

“For the time being we are satisfied with the way things are going under the circumstances, but we will continue our struggle to free our activists and have the case against them closed altogether. They didn't commit any crime, even hooliganism.”

Mikhail Kasyanov, a Russian opposition leader and a former prime minister under President Vladimir Putin, said he expected the activists to be released soon.

“In this way Russia is just sending a message: Don't meddle in our affairs,” Kasyanov said in an interview with Echo of Moscow radio station. “It is very much the style of our president.”

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sergei.loiko@latimes.com