From Longman Dictionary of Contemporary Englishbe on the casebe on the casespoken if someone says they are on the case, they know about a problem and are going to try to solve it → case
Examples from the Corpus
be on the case• And the National Guard were on the case now.• But Jack MacFarland was on the case.• Around 50 police officers are on the case.• And Rathbone would be on the case, with his bloodhound and magnifying glass. 