You're mute for the same reason that Mr.Whitmore tried to hide the fact that he was murdered! He thought he could keep Mrs.Collins's name out of the newspapers.

He wanted to save her from scandalous references involving her character! But you see how futile all his efforts were! You see how useless his self-inflicted torture was! Beard, look at this girl!" Britz pointed dramatically toward Miss Burden.

"You're engaged to her.

You've got a great deal to look forward to! But unless you get up now and tell the truth, the whole truth, concerning Mr.Whitmore's death, I promise you that the next ten years of your life shall be spent in the Federal prison at Atlanta." Beard shot a look of appeal at Luckstone, but the lawyer remained unmoved.
"You're making a rash promise, officer!" Luckstone scoffed.
"Beard!" Again Britz addressed himself to the frightened secretary.
"You've committed a grave crime.

You brought about the escape of a Federal prisoner--a man convicted of a serious offense.