Investors were licking their wounds on Tuesday as US stocks drifted up and down throughout the day after seeing the worst losses in some six years the day before.

Analysts spent the day discussing the possible reasons for the largest single point drop in the history of the Dow Jones Industrial Average (DJIA) on Monday, with many coming to the conclusion that algorithms were to blame.

While much of the selloff Monday could be seen as perfectly rational, a brief stretch before the close when the market tanked almost 1,000 in less than 20 minutes was what really spooked investors.

“The drop in the morning was caused by humans, but the free-fall in the afternoon was caused by the machines,” Walter Hellwig, senior vice president of BB&T Wealth Management told Bloomberg.

Volatile trading saw the main US benchmark indices seesaw throughout much of the morning, but the Dow was up much of the day despite opening down 500 points. The index closed up 2.33%, while the S&P 500 (+1.74%) and Nasdaq (+2.33%) followed suit.