TOKYO/WASHINGTON (Reuters) - Japan halted imports of U.S. beef on Friday just a month after lifting a ban, following the discovery of spinal material in a shipment that should have been removed due to the risk of mad cow disease. U.S. officials immediately launched an investigation and ordered extra training for all American meat inspectors, surprise inspections at plants handling beef exports, and sent a team of experts to examine meat shipments now held in Japanese ports. "This is an unacceptable failure on our part to meet the requirements of our agreement with Japan," Agriculture Secretary Mike Johanns told reporters....