By Valerie Insinna, David Shepardson and Rajesh Kumar Singh WASHINGTON/CHICAGO (Reuters) -Boeing’s latest 737 MAX crisis deepened after United Airlines and Alaska Airlines said they had found loose parts on multiple grounded MAX 9 aircraft, raising new concerns among industry experts about how its best-selling jet family is manufactured. U.S. regulators grounded 171 MAX 9 planes after a panel blew off an Alaska Airlines-operated flight not long after taking off from a Portland, Oregon, airport on Friday, forcing pilots to scramble to land the plane safely. Alaska Airlines said late on Monday t…