• Tower Comics never registered its titles with the US copyright office, nor did it include a correct copyright notice on their books. The copyright notice on the first issue was not in the proper location of the book which under copyright law at the time of it’s publication had to be “either upon the title page or upon the first page of text of each separate number or under the title heading.” Secondly, it was hidden in the artwork, which goes against the part of the law that stipulates that “The notice should be permanently legible to an ordinary user of the work under normal conditions of use and should not be concealed from view upon reasonable examination.” According to US copyright law, all works published between 1923-1977 that did not comply with copyright law became public domain upon publication. So, because the first issue had an incorrect notice, the characters fell into the public domain.
  • Only the Tower Comics version of the T.H.U.N.D.E.R. Agents is in the public domain. All subsequent versions published by DC Comics, JC Comics, Deluxe Comics, etc. areNOT. Also the T.H.U.N.D.E.R. Agents are curretly trademarked byJohn Carbonaro's estate

via  http://pdsh.wikia.com/wiki/T.H.U.N.D.E.R._Agents