In the opening sequence when Charlie discovers that his sons had been going through his mail, he states that he could forgive a poor report card but not the flattery and "...tampering with United Stated mail." Once mail is removed from the receiving mailbox it no longer carries the protection of the US Post Office (now the US Postal Service.) As a police official, Charlie would know that.
When Charlie and the authorities hear Mrs. Pendleton scream from the next room, they beat on the locked door and the set walls quiver.
Don Beddoe's character is referred to in the film as Frederick Ross, but the credits list him as James Ross