Complex sentence: Most of the casualties, however, occurred at the central market, which was filled with hundreds of shoppers at the time of the attack, not limited to Chechens, but also including many ethnic Russians and others.
Split into simpler sentences: Most of the casualties occurred at the central market, which was filled with hundreds of shoppers at the time of the attack. They were not limited to Chechens, but also included many ethnic Russians and others.