‘Tis the question. At least both the FSB and the Chechens appear to want to agree that the metro bombings were inspired as an act of revenge, though revenge for what exactly is less clear.
In his most recent video address, dated March 29, Umarov said the two suicide bomb attacks in the Moscow metro earlier that day were undertaken on his orders and constituted “a legitimate act of revenge” for the killing in early February by police and security forces of a group of impoverished Chechen villagers gathering wild garlic in a wooded area on the Chechen-Ingushetian border. Russian human rights activists confirmed that the men had been shot not in a combat operation but at close range, and their bodies mutilated.
From Irini Borogan and Andrei Soldatov in the Moscow Times:
It surely took the terrorists more than a month to prepare for the metro bombings. After all, the organizers had to bring the suicide bombers to the capital, obtain the explosives and select a secret apartment as a safe house. It thus seems clear that the metro bombings were not a reaction to the March killings of separatist leaders as the FSB has claimed.