Wall Street returns from vacation season to enter what is historically the most perilous period of the year amid conflicting signals about the US economic outlook.
Trade over the past week was choppy as investors reacted to various data that pointed toward both economic weakness and strength.
In the week to Friday, the blue-chip Dow Jones Industrial Average gave back 0.72 percent to finish the week at 11,543.96.
The broad-market Standard & Poor's 500 index lost 0.73 percent to 1,282.83 and the technology-heavy Nasdaq composite shed 1.95 percent to 2,367.52.
The main indexes closed lower for the week but held onto gains for a strong August. The Dow rose 1.92 percent, the Nasdaq 2.44 percent and S&P index 1.78 percent in the month.
The market was set to reopen Tuesday after the Labor Day holiday, commencing what is traditionally a treacherous month.
"Many investors believe October is the worst month for equity market returns. This can be partly attributable to the fact some large one-day declines have occurred in October," said David Templeton of the financial website Seeking Alpha.
"In actuality though, the worst month for market returns is September. Although the average return in September is negative, the magnitude of the decline was no worse than one to 1.5 percent."
Cross-currents affecting the market in recent weeks included up-and-down economic data and volatile oil prices.