The S&P 500 index Monday finished at a fresh 52-week low as a broad-based sell-off in the equity market gathered steam ahead of a key Federal Reserve meeting. The S&P 500 SPX, -0.18% closed 2.1% lower at 2545.94, marking its lowest finish since Oct. 9, 2017, according to Dow Jones Market Data. The finish put the broad-benchmark below its closing nadir in 2018 set on Feb. 8 when it concluded the session at 2,581. All 11 sectors of the index ended in the red. Market technicians view a decline below a recent low as a bearish sign, but it could also signal that the index is close to putting in a bottom. The Fed is slated to commence its two-day policy meeting on Tuesday, with investors eager to glean more intelligence about the pace of rate increases.