Bears appeared to tighten their grip on the market on Monday, with the BSE Sensex breaking the 15,000 levels to reach 14,926, shedding 645 points, or 4 per cent. The 50-share Nifty, also stooped to new lows to hit the Jan 22 low of 4448 levels. It is trading at 4423 levels, losing 204 points or 3.3 per cent.