A massive fire that destroyed Eugene's historic Civic Stadium was caused by four boys playing with fire who fled when the flames got out of control, investigators said Thursday.

The panicked boys jumped a chain-link fence as the flames spread Monday night, hopped on their bicycles and fled, said arson investigator Steve Williams.

The parents of a fifth boy who left before the fire tipped off police on Wednesday.

The four are to be charged with arson, Police Chief Pete Kerns said at a news conference Thursday. One is 10, the others 12.

The stadium was a jobs project in the Great Depression but hasn't been used since 2009 when the minor-league Eugene Emeralds left. A group determined to keep it from being demolished for a supermarket had bought it, taking out a $3 million insurance policy.

The boys probably didn't intend to destroy the park when they used a lighter on debris in the press box above the stands behind home plate, Williams said.

But they haven't given coherent reasons for why they were playing with fire, and they may not understand the magnitude of their actions, he said.

"I don't know if you've ever talked to a 10- or 12-year-old boy recently, and you don't get a whole lot of rational information out of them," Williams said. "It's: 'Why did you do that?' 'I don't know.'"

The fire sent flames shooting twice the height of the structure, and smoke was visible throughout the city, which is about 100 miles south of Portland.