<p>May 13, 2004 -- WASHINGTON - U.N. investigators have arrived in Baghdad to look at "smoking gun" files that purportedly document wholesale bribery of U.N. officials and international political figures by Saddam Hussein, The Post has learned. Sources close to the probe said staffers of the U.N. commission, headed by former Federal Reserve Board Chairman Paul Volcker, were in Iraq to open a critical phase of the investigation into the oil-for-food program scandal.</p>