NEW YORK – The price of oil plunged nearly 9 percent Thursday to settle below $100 per barrel. Investors who had ridden a months-long rally fled the market because of concerns about weakening demand for fuel in the United States.

Oil futures settled at $99.80 on the New York Mercantile Exchange. The decline of $9.44 per barrel, or 8.6 percent, brought the week’s loss for oil to $14.13, or 12.4 percent.

Oil rose 35 percent from mid-February through the end of April. As it climbed above $100, economists warned that high fuel prices were taking a toll on the U.S. economy.

Gasoline demand starting falling in March as motorists paid more at the pump. That trend was reinforced by industry and government studies released this week.

“More and more people were saying that oil was just too high,” said Michael Lynch, president of Strategic Energy & Economic Research. “That got a lot of investors ready to run for the door. That’s what they’re doing now.”

Analysts also said the lack of any terrorist retaliation for the killing of Osama bin Laden eased concerns about the safety of the world’s oil fields.

Oil and other commodities have been on a roll since about Labor Day, when the Federal Reserve indicated it would take more steps to boost the U.S. economy.