Why do Devine and his partner become suspicious of Soneji in the novel, Along Came a Spider?

When Devine and his partner witness Soneji canvassing the home of the Secretary of the Treasury, they begin to follow him to make sure he has no ideas about injuring the secretary or his family. During this investigation, Devine and his partner follow Soneji to the farm where he will later hold the children. However, Devine and his partner do not understand the gravity of this situation until after Soneji takes the children.