He established headquarters at Mound City and for a time quelled the forays, but after his force was disbanded trouble broke out afresh and it was then that James Montgomery took the field in defense of the frightened free-state settlers, and ordered the pronounced leaders of the pro-slavery party out of the county. Many of them obeyed the summons and moved with their families to Missouri.

Around Trading Post located on the Marais de Cygnes River, a bitter pro-slavery settlement had grown up, the leader of which was Charles A. Hamelton. The post thus became the rendezvous of the abolition haters not only for the immediate vicinity but for the territory across the line. Montgomery determined to break up this gang. He began by emptying the contents of several barrels of whiskey on hand at the "doggery," and leaving a notice for the ruffians to quit Kansas Territory. Hamelton and some of his neighbors left the territory. Subsequently they called a meeting at Papinsville, Missouri to incite the men to an invasion of Kansas. Hamelton addressed the meeting and with a unanimous vote, it was decided to invade the territory and exterminate the free-state settlers in Linn County.

When the party arrived at the line between Missouri and Kansas a halt was ordered to make final arrangements. One of the men named Barlow, who had spoken against the invasion at the meeting, again did so, and this time with better effect. They were on the border of the hated but also dreaded Kansas, and Barlow assured them that the crack of the Sharpe's rifles might be expected from Montgomery's men at any minute. A panic seemed imminent, but at the summons of Hamelton about 30 of the most resolute rode after their leader and reached the post on the morning of May 19, 1858, where they captured one man and then proceeded on the road toward Westport, Missouri capturing three more on the way. The next capture was Austin Hall and his brother Amos. In all, eleven men were taken prisoners, nearly all of whom were known to Hamelton or some member of his party. They were not known to have taken any active part in the disputes, and having been neighbors of Hamelton they had no suspicion that he meant to harm them, especially as they were guilty of no offense but that of being free-state men. The eleven victims were driven at a rapid pace into a deep gorge, where they were lined up facing east. Hamelton then ordered his men to form in front of them and fire. One of the men turned out of the line and refused to do so, but Hamelton brought the remainder into line and fired the first shot himself. Six of the victims were not killed and the men fired at them again. One man, Austin Hall, was not touched, but feigned death and thus escaped. The dead were carried to the Trading Post and the wounded cared for. The State of Kansas later appropriated $1,000 for a memorial monument, which was erected at the Trading Post, beneath which rested the ashes of Colpetzer, Campbell, Ross and Robinson. The body of Stilwell was taken to Mound City for burial.