The collapse in oil prices over weekend added uncertainty coming into this week.
By the end of Monday, the S&P 500 had posted its largest percentage point decline since 2008. The index closed at its lowest level since June.
Evidence of expanding pessimism could be seen in the options market. The VIX fear index soared to its highest close since 2009 and the CBOE put/call ratio finished the day near the all-time high reached in December 2018.