Fire department investigators have confirmed that the blaze that tore through a packhouse owned by Washington apple shipper Evans Fruit Co Inc more than a week ago was the work of arsonists.

Two of the firm’s three packhouses are shipping apples as usual, but the firm’s third packhouse in Tieton, about 15 miles west of Yakima, Washington, was severely damaged by the arson fire.

“We lost a loading dock, a controlled-atmosphere room and maybe all of our computer-controlled packing lines,” a company spokeswoman told the local press. “But we’ll be back in business in about three weeks.”

Fire department officials estimate the loss at $5 million (£2.5m). The department has offered a $10,000 reward for information leading to the arrest and conviction of the person or persons responsible for the fire, which broke out in the early hours of April 24, the company spokeswoman said. Two men were reported running from the scene by a neighbour.

The other two packhouses are expected to pick up the workload of the third packhouse.

Evans Fruit is among Washington’s largest apple grower-shippers, with nearly 7,000 acres of orchards.