The Case of the Bank-robbing Bandit (Third-grade Detectives)

IT'S DO OR DIE FOR THE THIRD-GRADE DETECTIVES!
Todd and Noelle are volunteering at a blood drive when they get shocking news: Both banks in town have been robbed! This may be the Third-Grade Detectives' most important case yet. The new police chief doesn't think he needs help from a bunch of third graders to solve crimes, and if the kids can't prove him wrong, their detective careers may be over.
The trouble is, the thief hasn't left any clues, except for a discarded sweaty handkerchief. Yuck! Can something so disgusting provide the lead the Detectives need?