As firefighters worked to contain the fire, which had burned more than 5,000 acres since it was started Tuesday, officials said they had not determined the motive of whoever was responsible for setting it and four others, the St. Louis Post-Dispatch reported Sunday.
"I don't know what their mindset is -- if they are bored or liquored up, if they have a personal beef," said Thomas Haines, a U.S. Forest Service ranger for the area. "We could speculate all day."
Officials say three fires were set Tuesday, with one becoming the Mark Twain forest's largest fire in history. A day later, at least two more fires were started in nearby Reynolds County, burning about 300 acres.
Bunker Mayor Linda Vest says arson is a problem every year around this time, but is puzzled as to why.
"Why would anyone start a fire?" she asked. "Once you set a fire, you don't know what direction it's going to go, whose house or property could be in its path. There could be children in the path."
Although officials have few leads, Haines said he suspects only one or maybe two people are to blame.
"Trust me, I go to church with these people, I go to work with these people, and it's one yahoo that makes it possible to paint everybody bad," he said. "If we catch him, it's going to be because somebody in the community helped us do it."