The S&P 500 fell from its recent record Monday as Omicron variant worries reignited ahead of the Federal Reserve's FOMC monetary policy decision later this week. The S&P 500 fell nearly 1%, while the Dow Jones Industrial Average dropped over 300 points and the Nasdaq slumped over 200 points.

Here's how the market settled to start the week:

S&P 500 Index (SPY  ): -0.91% or -43.04 points to 4,668.98

Dow Jones Industrial Average (DIA  ): -0.89% or -320.04 points to 35,650.95

Nasdaq Composite Index (QQQ  ): -1.39% or -217.32 points to 15,413.28

OSHA opens investigation into deadly Amazon warehouse collapse:

The Occupational Safety and Health Administration (OSHA) has opened an investigation into a deadly Amazon (AMZN  ) warehouse collapse in Edwardsville, Illinois, an agency spokesperson Scott Allen told CNBC on Monday. The collapse was caused by a major tornado on Friday.

OSHA has six months to complete its probe, issue citations and propose monetary penalties if violations of workplace safety and/or health regulations are found, Allen told CNBC. The agency has had compliance officers at the warehouse since Saturday to provide assistance, Allen added.

Amazon spokesperson Kelly Nantel told CNBC in a statement: "OSHA investigates all workplace fatalities and we are supporting them."

USPS expects 2.3 billion deliveries this week:

The U.S. Postal Service announced Monday that it expects 2.3 billion pieces of mail to be delivered during the week of Dec. 13, demonstrating the high consumer shipping demand this holiday season. The USPS estimate includes both letters and packages.

"Since Dec. 6, customer traffic at all Post Office locations has been steadily increasing," the USPS said in a press statement. "But this week is expected to be the busiest week of the holiday mailing and shipping season."

The USPS estimates that 850 million to 950 million packages are expected to be delivered between Thanksgiving and New Year's Day.

Here's how market benchmarks started trading soon after market open:

S&P 500 Index: -0.11% or -5.26 points to 4,706.76

Dow Jones Industrial Average: -0.08% or -5.26 points to 35,943.90

Nasdaq Composite Index: -0.23% or -35.93 points to 15,591.80