Citigroup Sued Over Enron Securities  NEW YORK (Reuters) - Citigroup Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=C.N target=/stocks/quickinfo/fullquote"&gt;C.N&lt;/A&gt; faces a lawsuit  from angry investors who allege they were defrauded in a  "massive scheme of deception" when they bought securities tied  to the credit-worthiness of bankrupt energy trader Enron Corp.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=ENRNQ.PK target=/stocks/quickinfo/fullquote"&gt;ENRNQ.PK&lt;/A&gt;.