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What a way to start the first full week of trading in 2015. The Dow Jones Industrial Average tumbled more than 300 points on Monday, dragged down by falling oil prices amid worries that while they are a boon for consumers, they could hurt other sectors of the economy.

It was the worst session for the Dow and for the broader S&P 500 index in over three weeks. The Dow unofficially dropped 1.84 percent, the S&P shed 1.81 percent and the Nasdaq dropped 1.6 percent. Monday capped the stock market’s first four-day losing streak since December 2013 and sent stocks to their lowest level in two weeks.

The Dow last closed at a record high 18,053.71 on Dec. 26; the S&P 500 finished at a record 2,090.57 three days later.

At the rout’s forefront was oil, which dropped to its lowest level since April 2009 on a growing supply glut and slipping demand. While American consumers have been reveling in the lowest gas prices in years, investors have grown increasingly worried about the effects of lower oil prices on energy company profits and on countries, such as Russia, whose economies are heavily dependent on oil exports.

U.S. crude prices slumped below $50 a barrel while benchmark Brent crude tumbled under $53 after data showed Russian output at post-Soviet era highs. Stocks for energy companies tumbled. Chevron fell $4.4, or 3.9 percent, to $108.15. Exxon Mobil dropped $2.76, or 3 percent, to $90.07.

"We're starting a new year with a bit of trepidation; it's difficult for equities to get any traction when one of the asset classes is in a free fall," said Art Hogan, chief market strategist at Wunderlich Securities, referring to crude's ongoing decline.

Concerns about Europe also weighed down shares after reports emerged that German Chancellor Angela Merkel no longer believes it would be too risky for the 19-member eurozone if Greece dropped out of the currency bloc.

How low can oil go?

Jan. 5, 201501:18


-- NBC News staff, CNBC and The Associated Press