In Chapter 23, how does Detective Marshall discover that Duane Jr. might not actually be the suspect he is looking for?

In Chapter 23, Detective Jason Marshall's determination to find Duane Jr leads him to the store that sold the butane blowtorch that was found in Duane's bag. While watching the surveillance video, he realizes that Duane Jr was not the buyer. He then calls the arson investigator and suggests that they are looking for the wrong person.