The aggregation of highly mobile carbon interstitials at carbon antisites and as interstitial clusters is investigated by an ab initio method. The aggregates act as a source of interstitials at high temperatures and are important for the formation of stable defect centers such as D-I and D-II. The analysis of local vibrational modes indicates that (C-2)(Si) is a building block for the D-II center. We also discuss possible candidates for the P-T centers.