  The low temperature state of CeRu2Al10 has been studied by neutron powder
diffraction and muon spin relaxation (muSR). By combining both techniques, we
prove that the transition occurring below T*~27K, which has been the subject of
considerable debate, is unambiguously magnetic due to the ordering of the Ce
sublattice. The magnetic structure with propagation vector k=(1,0,0) involves
collinear antiferromagnetic alignment of the Ce moments along the c-axis of the
Cmcm space group with a reduced moment of 0.34(2)mu_B. No structural changes
within the resolution limit have been detected below the transition
temperature. However, the temperature dependence of the magnetic Bragg peaks
and the muon precession frequency show an anomaly around T2~12 K indicating a
possible second transition.
