Electron spin resonance produced in silicon by fast neutron irradiation was studied. The temperature of the samples during the irradiation was about 50°C. Different spectra were observed depending on the Fermi level in the irradiated sample. Samples with the Fermi level near the middle of the energy gap were studied in some detail. The observed complex spectrum was independent of the content of chemical impurities including oxygen. The spectrum simplified after annealing at temperatures >=170°C. A center designated Si-N was identified in the spectrum of the annealed samples. The character of the g tensor and the hyperfine interaction suggest that at T<=77°K the unpaired electron is in a broken bond orbital on a single silicon atom. Variation of the spectrum with temperature was studied. The results may be explained by assuming that the unpaired electron has the choice of more than one broken bond orbital in the defect center.