By Deborah Zabarenko WASHINGTON, May 28 (Reuters) - Two truck-trailers found in Iraq were "ingeniously simple" mobile biological weapons factories, with other as-yet-undiscovered trailers holding the end of the production chain, the CIA said on Wednesday. No trace of biological weapons has been found in either of the trailers, but there is little question they were constructed to make such toxins as anthrax and botulinin in quantities that potentially could kill thousands of people, U.S. intelligence officials said in telephone conference called to discuss a new report by the CIA and Defense Intelligence Agency. "Our experts who are in the...