Although Mr. Maskhadov , who was 53 , nominally commanded Chechnya 's fighters , he appeared to have lost influence over Russia 's most wanted man , Shamil Basayev , the rebel commander who has claimed responsibility for the worst terrorist attacks , including the siege of a theater in Moscow in 2002 that killed 129 and a school in Beslan last September that killed at least 339 , half of them schoolchildren .