(Bloomberg) -- The sell-off that sent the S&P 500 Index to its biggest plunge since February is likely to worsen. Futures on the gauge sank as much as 1.1 percent to 2,750.25 in early trading Thursday, a level that is lower than the S&P 500’s 200-day moving average. The benchmark index last dipped below the technical level in May, before staging a rebound that took it to a record last month.