A previously unknown Chechen rebel group has claimed responsibility for the bombing on the Moscow metro last month that killed at least 40 people.
The 'Gazotan Murdash' group claimed responsibility in a statement released to a website.
"Our first operation was successfully conducted on February 6 on the Moscow metro," the statement said, saying it was revenge for an alleged atrocity by Russian soldiers in Chechen capital Grozny exactly four years before the metro blast.
According to human rights group Memorial, 46 people died when Russian troops stormed through the Alda suburb in February 2000 as they consolidated power in mainly Muslim Chechnya after their return to the region in 1999.
The statement, signed by Lom-Ali Chechensky, went on: "It was small, but good. Remember! This was revenge for Alda February 5-6, 2000, and it is only the beginning. An eye for an eye! An injury for an injury! Freedom or Death!"
Chechen rebels gained de facto independence from Russia in 1996, but Russian forces poured back three years later. The conflict has degenerated into a bitter guerrilla war, which claims the lives of Russian troops daily.
Extreme separatists have increasingly turned to suicide bombings - killing more than 300 people last year - but the statement made no mention of the metro bombing having been conducted by a suicide bomber.
After the bombing, President Vladimir Putin immediately blamed Chechen rebels, but until now rebels had distanced themselves from the blast.
In an editorial comment, Kavkazcenter, which is linked to the extreme wing of the rebel movement, said that if this group was truly independent of established Chechen leaders, it showed a new development in the conflict.
"If this statement proves to be true, and if a new previously unknown group stands behind the acts on the Moscow Metro, then the war between Russia and Chechnya has entered a qualitatively new phase when an absolutely autonomous group bent on revenge appears on the scene," said the comment.