[OS X TeX] bibliography in letter document class

[Samuel H Cox, Jr.]

How can I add a bibliography to a letter?  I have tried both scrlttr2 and letter document classes.  I used this example and got this error from the scrlttr2 version and similar from the letter version:

%!TEX TS-program = pdflatexmk

\documentclass[fontsize=12pt, paper=letter]{scrlttr2}
\usepackage[longnamesfirst]{natbib}

% Dont forget to read the KOMA-Script documentation, scrguien.pdf

\setkomavar{fromname}{} % your name
\setkomavar{fromaddress}{Address \\ of \\ Sender}

\setkomavar{signature}{} % printed after the \closing
\renewcommand{\raggedsignature}{\raggedright} % make the signature ragged right

\setkomavar{subject}{} % subject of the letter

\begin{document}
\bibliographystyle{apalike}
\begin{letter}{Name and \\ Address \\ of \\ Recipient}

\opening{}  % eg. Hello

some stuff

\closing{} %eg. Regards

\bibliography{unified}
\end{letter}

\end{document}

....

)) (/usr/local/texlive/2014/texmf-dist/tex/latex/koma-script/DIN.lco)
(/usr/local/texlive/2014/texmf-dist/tex/latex/natbib/natbib.sty

/usr/local/texlive/2014/texmf-dist/tex/latex/natbib/natbib.sty:1063: LaTeX Erro
r: Environment thebibliography undefined.

See the LaTeX manual or LaTeX Companion for explanation.
Type  H <return>  for immediate help.
 ...



l.1063 \renewenvironment{thebibliography}
                                         [1]{%
?
Process aborted




Sam
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