Moscow - A Russian judge Friday ordered state investigators to reopen the probe into the murder of investigative journalist Anna Politkovskaya, after prosecutors failed to convince a jury that four men accused had a part in her killing, Russian news agencies reported.

"The criminal investigation will be sent back to the Russian Prosecutors' Investigative Committee to find the individuals who committed this crime," the presiding judge at the Moscow military court, Yevgeny Zubov, was quoted by Interfax as saying.

On Thursday a 12-member jury of fully acquiited four men men linked by state investigators to Politkovskaya's death of any involvement in the crime.

The drawn-out state investigation and chaotic trial of her murder has from the first been criticized as a farce by Politkovskaya's family and friends because the men on trial were only loosely charged with guilt in the crime.

Freed from blame were two Chechen brothers Dzhabrail and Ibragim Makhmudov of being accomplices and former police officer Sergei Khadzhikurbanov of helping the killer get away.

A fourth defendant, Pavel Ryaguzov, was acquitted in a separate case. Ryaguzov, an agent of Russia's FSB security service was accused of providing the killer with Politkovskaya's address.

But the alleged gunman, a third Chechen brother Rustam Makhmudov, was never on trial and reportedly fled to Western Europe on a fake passport.

No-one has been directly linked to ordering the contract-style killing of Politkovskaya, gunned down in front of her Moscow apartment in October 2006.

Politkovskaya was Russia's most prominent journalist, winning awards for her reporting for daily Novaya Gazeta on human rights abuses by Russian soldiers during tow wars fought by the Kremlin in Chechnya.

Her killing sent shock waves through the international community and raised concerns about crackdowns on Kremlin-critical reporters under former president Vladimir Putin, now serving as prime minister.

The US Embassy in Moscow voiced concern on Friday that Politkovskaya's murder remains unsolved over two years after her death.

"We sincerely hope that the Russian authorities will find those responsible and bring them to justice," the embassy said in an e-mailed statement.

Last month, another Novaya Gazeta journalist Anna Baburova and a prominent rights lawyer were gunned down in central Moscow last month. The newspaper has lost five journalists to violent deaths since 2000. (dpa)