Boeing's latest 737 Max crisis deepened on Monday after United Airlines said it had found loose bolts 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.

United, one of the two U.S. carriers that fly this Boeing model with the panels, said its own preliminary checks found bolts that needed tightening on several panels. That disclosure heightened concerns about the production process of the Max 9 jets that have been grounded.