The Dow Jones Industrial Average points dropped 25.58 points (0.20 percent) to 12,480.18 in closing trade.
The tech-rich Nasdaq Composite points rose 5.72 points (0.20 percent) to 2,825.88, while the broad-market S&P 500-stock index shed 2.09 points (0.16 percent) at 1,335.29.
"This week brings the first Fed press conference Wednesday following its two-day meeting and the earnings season kicks into high gear," said Scott Marcouiller at Wells Fargo Advisors.
Investors could tread cautiously ahead of the two-day Federal Open Market Committee (FOMC) meeting that opens Tuesday, where the central bankers are widely expected to maintain their policy of rock-bottom interest rates, said Andrea Kramer at Schaeffer's Investment Research.
Ryan Sweet at Moody's Analytics said the pressure would be on chairman Bernanke to clarify the FOMC's position on growth, inflation and monetary policy when he takes questions from the media after the meeting.
"Because this will be Bernanke's first post-FOMC press conference (and the first for any Fed chairman), there is little room for error. Miscommunication could rattle financial markets and create additional uncertainty about the path of monetary policy," Sweet said in a research note.