Who is Mr. Frederick Johnson in "In Cold Blood"?

Frederic Johnson
(note the correct spelling of the first name) is the brother-in-law of Perry Smith, one of the
two men convicted of murdering four members of the Clutter family. His wife, Barbara, was
Smith's only surviving sibling.

He is mentioned
twice:

1. Capote provides the text of a letter to Smith written and
   signed by Barbara and Frederic.Capote indicates to readers that he will refer to Barbara as Mrs.
   Frederic Johnson.

   (In the latter instance, Capote may be implying that
   "Johnson" is not the couple's last name, perhaps to protect their true
   identity.)