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A fire burning on Dugway Proving Ground swelled to more than 4,000 acres Saturday and state fire officials say the blaze is the largest they\'ve seen this year.

DUGWAY PROVING GROUND — Firefighters were working to contain a fire that ignited Wednesday morning near the Dugway Proving Ground.

Officials were notified at 10:07 a.m. after the Pony Fire ignited on the southern border outside of the proving ground near Simpson Springs in Tooele County.

The fire was "actively burning and spreading quickly," according to Dugway spokeswoman Paula Thomas.

The fire had burned about 1,500 acres and was 40 percent contained by Wednesday afternoon.

Dugway firefighters initially responded to the fire before turning it over to Bureau of Land Management resources. Two single-engine air tankers, five engines and four 20-person hand crews were on the scene Wednesday afternoon. Two additional air tankers were also on order.

The cause of the fire was not immediately known.

— Morgan Jacobsen