BERLIN/WASHINGTON (Reuters) – Volkswagen (VOWG_p.DE) shares plunged by nearly 20 percent on Monday after the German carmaker admitted it had rigged emissions tests of diesel-powered vehicles in the United States, and U.S. authorities said they would widen the probe to other automakers. German officials, alarmed at the potential damage the scandal could inflict on its car industry, urged Volkswagen to fully clear up the matter and said it would investigate whether emissions data had also been falsified in Europe.