Anderson's sisters aided the guerrillas by gathering information inside Union-controlled territory. In August 1863, however, Union General Thomas Ewing, Jr. attempted to thwart the guerrillas by arresting their female relatives, and Anderson's sisters were confined in a three-story building on Grand Avenue in Kansas City with a number of other girls. While they were confined, the building collapsed, killing one of Anderson's sisters. In the aftermath, rumors that the building had been intentionally sabotaged by Union soldiers spread quickly; Anderson was convinced that it had been a deliberate act. Biographer Larry Wood wrote that Anderson's motivation shifted after the death of his sister, arguing that killing then became his focus, and an enjoyable act. Castel and Goodrich maintain that by then killing had become more than a means to an end for Anderson: it became an end in itself. Although Quantrill had considered the idea of a raid on the pro-Union stronghold that was the town of Lawrence, Kansas before the building collapsed in Kansas City, the deaths convinced the guerrillas to make a bold strike. Quantrill attained near-unanimous consent to travel into Union territory to strike Lawrence. The guerrillas gathered at the Blackwater River in Johnson County, Missouri. Anderson was placed in charge of 40 men, of which he was perhaps the angriest and most motivated—his fellow guerrillas considered him one of the deadliest fighters there. On August 19, the group, which proved to be the most guerrillas under one commander in the war, began the trip to Lawrence. En route, some guerrillas robbed a Union supporter, but Anderson knew the man and reimbursed him. Arriving in Lawrence on August 21, the guerrillas immediately killed a number of Union Army recruits and one of Anderson's men took their flag. The Provost Marshal of Kansas, a Union captain who commanded military police, surrendered to the guerrillas and Anderson took his uniform (guerrillas often wore uniforms stolen from Union soldiers). They proceeded to pillage and burn many buildings, killing almost every man they found, but taking care not to shoot women. Anderson personally killed 14 people. Although some men begged him to spare them, he persisted, only relenting when a woman pleaded with him not to torch her house. The guerrillas under Anderson's command, notably including Archie Clement and Frank James, killed more than any of the other group. They left town at 9 AM after a company of Union soldiers approached the town. The raiding party was pursued by Union forces but eventually managed to break contact with the soldiers and scatter into the Missouri woods.