Knit Some Deaths — David Williamson

A fatal stabbing with a knitting needle drags Grandpa and Jimmy into a police investigation. Detective Garcia finds them snooping around at the murder scene, but since they proved helpful on other cases they are permitted to help on this one. The investigation runs into dead ends, both of leads that go nowhere and finding additional dead bodies. Finally, at Grandpa's suggestion, Detective Garcia changes the direction of their efforts and searches for something to explain why the killer continues to strike.