Electron paramagnetic resonance, in conjuction with electrical measurements, has been used to study Si-SiO2 interface defects produced as a result of negative bias, high-field electron injection into the SiO2 layer of metal-oxide-semiconductor capacitors. In particular, a search was made for Pb centers, previously identified as a source of interface traps in unannealed SiO2 on Si, and for E' centers, which have been associated with trapped holes in SiO2. The results of the EPR experiments show no change in the density of Pb centers following injection, despite the presence of interface traps as indicated by the electrical measurements. Further, no E' centers are detected in samples for which an analysis of the current-voltage and capacitance-voltage data suggest the presence of 5×1012 cm-2 positively charged centers. We conclude that interface traps generated in the present samples are not of the same microscopic nature as those found in unannealed SiO2 on Si, and that positive charging under the present conditions is not due to trapped holes. It is suggested that the source of positive charge is the so-called "anomalous positive charge" (APC) center. The microscopic nature of the present defects has not been identified.