No serious injuries were reported in Tuesday night's blaze at the Thomson plant in Marion, about 60 miles northeast of Indianapolis.
The fire caused an estimated $750,000 in damage, mostly to inventory, authorities said.
Fire Chief Steve Gorrell said evidence found at the scene confirmed the blaze was arson. He declined to disclose details, citing the ongoing investigation.
``We know how it started. It's just a matter of finding out who started it,'' he said.
The plant supplied picture tubes to Thomson plants in Mexico and to other manufacturers.
Some employees were still in the building during the fire, which came more than four hours after Thomson announced it was ending production at the plant, authorities said. The company blamed declining sales and a shift toward digital and plasma TVs.
Fire investigators were working with the company to obtain a list of employees who were in the factory at the time of the fire.
