The slave hunters question Huck and come very close to catching Jim. Huck is the only thing standing between Jim and capture. Right before he encounters the slave hunters, Huck has a "crisis of conscience" about how wrong it is to break the law, which is what he is doing by helping Jim to escape.
However, when the slave hunters question him, Huck lies and tells them that all those on board the raft have smallpox. The slave hunters don't want to search and risk catching this dreaded disease, so Jim is safe for now.
After the incident, Huck ponders his actions and decides that he's done something wrong, but if he'd turned Jim in, he'd feel just as bad. Twain's use of an uneducated child narrator offers the opportunity to highlight the immorality of slavery and contrast what the law of the land says with what God's higher law says.