BIXBY, Mo. (AP) -- Arson is the suspected cause of a fire that burned about 620 acres of the Mark Twain National Forest.The U.S. Forest Service said Monday that law enforcement is investigating. The blaze started Thursday morning in southeast Missouri's Iron County.About 45 miles to the east, a second fire started near Fredericktown after a mower hit a rock, causing a spark. That fire burned another 50 acres of Mark Twain National Forest lands.Containment lines completely surround both fires. The Forest Service says the goal is to have both blazes fully controlled by midweek.Currently, about 100 people are involved in the firefights. No injuries have been reported.Authorities say the drought and lack of rain have created an extremely high risk for fires and urged people to be cautious.