Probably in most such cases about half the witnesses do not testify at all.

Most of what has been said relates to the preparation for trial of cases where the accused is already under arrest when the district attorney is called into the case.

If this stage has not been reached the prosecutor may well be called upon to exercise some of the functions of a detective in the first instance.
A few years ago it was brought to the attention of the New York authorities that many blackmailing letters were being received bearing the name of "Lewis Jarvis." These were of a character to render the apprehension of the writer of them a matter of much importance.

The letters directed that the replies be sent to a certain box in the New York post-office, but as the boxes are numerous and close together it seemed doubtful if "Lewis Jarvis" could be detected when he called for his mail.

The district attorney, the police, and the post-office officials finally evolved the scheme of plugging the lock of "Lewis Jarvis's" box with a match.