What is the main conflict in Heaven's Prisoners by James Lee Burke?

Dave Robicheaux and his wife Annie inadvertently become involved in a underworld power struggle when the witness a small plane crash and rescue a young orphaned girl from the wreck. They take the girl home and Dave sparks the ire of an unknown criminal boss when he asks questions about the cause of the plane wreck. The situation spirals out of control and results in the murder of Annie during an attempted hit on Dave. Dave investigates as the criminals turn on each other until only one is left—whom Dave then arrests for murder.