I saw the last 20 or 30 minutes - pretty female prisoner being protected in an apartment before a trial. Cop starts to fall for her. She fights with sister, who doesn't want her to testify, sister's husband gets killed. Lots of shadows outside curtains, lots of men breaking in, lots of people shot. Cop dies near the end. Last line is the woman at the trial, testifying. When asked her occupation, she says 'Right now I'm a gangbuster' and the courtroom erupts.

(Pretty sure it was "Illegal")

I think you're looking for 'Marked Woman', here's a snippet of the plot that matches your description:

Mary's younger sister Betty (Jane Bryan) comes to visit, and unaware of the dangerous situation she has entered, behaves recklessly against the advice of her older sister. When she is killed, Mary agrees to testify against the gangster.

Marked Woman - Wikipedia, the free encyclopedia

That does sound similar to the short part I saw. But this movie had Edward G. Robinson as the lawyer, and then all the other cast fell into place. Thanks. Looks like another interesting film noir I never saw.

Oops! Guess I should read every thread next time!