The S&P 500 and FTSE 100 could plummet further if key resistance levels aren't held, Phil Roberts, technical analyst from Barclays Capital, told CNBC.
If the S&P 500 can't close above 986 and if the FTSE doesn't close above 4,347 then "the mayhem could still continue," Roberts told "European Closing Bell."
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The FTSE has enjoyed three consecutive sessions of gains this week, giving some investors hope for calmer markets ahead. But the VIX volatility index is still trending higher.