How is “be on the case” used?

In The Letter (1940), one line is: “Ong's been of great help on the case.”

Which movie is “be on the case” from?

The clip at the top of this page is from The Letter (1940).

to be dealing with a particular matter, especially a criminal investigation

Heard in 12 movies

Oxford83% of films
to be dealing with a particular matter, especially a criminal investigation
Good news. He's on the case.

Definitions from the Oxford Advanced Learner's Dictionary, 9th edition

Ong's been of great help on the case.

Said in 8 films · #1