Anna Politkovskaya was fatally shot in October 2006. (Lehtikuva, Marja Airio/Associated Press)

Prosecutors in Russia filed formal charges Wednesday against three men accused of involvement in the killing of crusading investigative journalist Anna Politkovskaya in 2006.

A fourth man, a federal police officer, who has been in custody since last August in connection with the killing, was charged with abuse of office and extortion, but prosecutors say those charges are in connection with other crimes.

The officer had earlier been accused of giving Politkovskaya's address to the man who allegedly killed her, but the charge sheet released by Russia's Investigative Committee makes no reference to that.

A fifth man named by authorities in Moscow last month as the person who actually shot Politkovskaya remains at large. He is the brother of one of the men charged Wednesday.

Colleagues believe her killing was linked to her work reporting on abuses by Russian troops in Chechnya, and three of the four men charged Wednesday are Chechens.

No explanation of death

The Investigative Committee statement gave no reason or motive for the attack on Politkovskaya in her central Moscow apartment building in October 2006.

Human rights groups and organizations that work for the protection of journalists say her killers may have been close to the government of then Russian president, Vladimir Putin,

Politkovskaya's colleagues at the Novaya Gazeta newspapers accused authorities of deliberately undermining the investigation by releasing details about the case before it reaches trial.

Novaya Gazeta editor Dmitry Muratov said Wednesday that leaks from investigators allowed the accused shooter to escape arrest, the Interfax news agency reported. Muratov called it too early to consider the murder solved.

Russian officials have alleged that Politkovskaya's killing was ordered by someone living outside Russia with the aim of discrediting the Kremlin — statements that have been interpreted as alluding to Boris Berezovsky, a former ally of Putin's who lives in London and is now a fierce Kremlin critic.

Berezovsky is wanted in Russia on embezzlement and other charges, all of which he denies and says are politically motivated.

He has said he had nothing to do with Politkovskaya's death.

Death damages Russia, Putin

Politkovskaya's work for Novaya Gazeta won her many awards, and she faced several unsuccessful attempts on her life before her murder.

On the day of her funeral in Moscow in 2006, Putin said her killing inflicted far more damage on Russia than her investigative work into abuse of office by his government's officials and security forces.

At least 12 other journalists have been killed in Russia in recent years, human rights groups say.

With files from the Associated Press