Benchmark Sensex slumped 260 points while the Nifty cracked below the crucial 10,000-mark on Wednesday as Sebi clamped down on 331 suspected shell companies listed on exchanges. Investors were spooked after the markets watchdog last night directed bourses to initiate action against the suspected shell companies. Brokers said the order raised concerns of more such regulatory action. A majority of these firms are apparently facing probe for alleged tax evasion and corporate frauds and have been referred by the I-T Deptt and SFIO to the corporate affairs ministry and Sebi for further action.