Analysts described investor unease after President Donald Trump's unsubstantiated accusations against former president Barack Obama dominated weekend headlines.
Market watchers also have said a pullback would not be surprising after a series of records since Trump's election in November.
About 55 minutes into trading, the Dow Jones Industrial Average stood at 20,939.39, down 0.3 percent.
The broad-based S&P 500 dropped 0.5 percent to 2,371.37, while the tech-rich Nasdaq Composite Index fell 0.6 percent to 5,833.92.
US shares of Deutsche Bank fell 3.1 percent after announcing it would raise cash by issuing new shares.
General Motors fell 0.9 percent after disclosing that it will take a charge of up to $4.5 billion mostly due to pension expenses associated with the sale of its European subsidiary to French carmaker PSA for 2.2 billion euros ($2.3 billion).
Delta Air Lines dropped 2.7 percent as it said revenues were coming in at the low end of expectations and higher costs were pinching profits in the current quarter.