A sharp pullback in the largest oil ETF on Monday afternoon had traders speculating on a “fat finger” trade and a potential release of the U.S. Strategic Petroleum Reserve.

U.S. Oil Fund (NYSEArca: USO) was flat most of the day but made a violent move lower just before 2 pm ET.

More than 21 million shares of USO had traded heading into the final hour, well above the 8.4 million average daily volume over the past three months.

Crude futures in New York plunged by nearly $5 a barrel in just a few minutes on Monday afternoon.

“It was not immediately clear what caused the crash, but traders said it could have resulted from a problem with a computer trading program,” Reuters reported.

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