AIG Says Unit Faces U.S. Criminal Probe  NEW YORK (Reuters) - American International Group Inc.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=AIG.N target=/stocks/quickinfo/fullquote"&gt;AIG.N&lt;/A&gt;, the world's largest insurer by market value, on  Wednesday said the U.S. Department of Justice opened a criminal  probe into one of its units over possible securities law  violations.