Oil plunged suddenly Monday afternoon, dropping more than $4 US per barrel at one point in a dramatic end to an otherwise quiet trading day in New York.
Benchmark crude fell $2.38, or 2.4 per cent, to finish at $96.62 a barrel on the New York Mercantile Exchange.
That's the biggest percentage decline since July 23. Oil plunged below $95 per barrel during the sell-off.
Traders were unsure of the cause of Monday's price drop.
Some questioned whether an errant trade or another rumour about a release of oil from the Strategic Petroleum Reserve was to blame.
The White House has been considering tapping the SPR to stem the rising cost of crude and gasoline.
A little over a week ago, Reuters reported the Obama administration was considering a release much larger than the 30 million barrels from last year.
Oil rose 2.7 per cent last week and touched a high above $100 a barrel after getting a boost from the Federal Reserve's latest plan to spur the U.S. economy.
Concerns that unrest in the Middle East over an anti-Islam film could trigger supply disruptions were a contributing factor.
Brent crude, which is used to price international varieties of oil, fell sharply as well. Brent lost $2.91, or 2.9 per cent, to $113.77 a barrel on the ICE Futures Exchange in London.