Electron spin resonance (ESR) was used to study neutron-induced defects in silicon as functions of anneal temperature T-an. For T-an below 200 degrees C, the ESR response is dominated by the Si-P3 and Si-P6 spectra, as observed before. At T-an=200 degrees C, a low symmetry center Si-H8, only once reported before, appears. Here, a hyperfine doublet, corresponding to interaction with one Si-29 nucleus, is resolved. In the range T-an>= 250 degrees C, two other paramagnetic defects dominate. The first is the tetragonal center Si-B3: Mapping of the full angular dependence of known and newly resolved Si-29 hyperfine structure enables conclusive identification as the tetra-interstitial (I-4) in the positive charged state, based on compliance of the data with former theoretical calculations. Second, we report the observation of an unidentified trigonal center, Si-B5, exhibiting two hyperfine (hf) doublets corresponding with hf interaction with four and six equivalent Si sites. The tri-interstitial I-3 is proposed as a provisional defect model. The annealing behavior and symmetry of these defects provides evidence for linking these paramagnetic B3 and B5 centers to the X and W optical centers, respectively.