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.

US 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 US 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.

A source familiar with the matter said United has so far found closer to 10 airplanes with loose bolts during its preliminary checks, up from an initial five first reported by industry publication The Air Current, and the figure may increase.

There are still ongoing discussions between Boeing, the US Federal Aviation Administration (FAA) and the airlines on the precise inspection guidelines.

Boeing is expected to revise the guidelines it submitted to airlines earlier on Monday, and the FAA would have to sign off on those changes before the airlines could begin repairs, sources said.

Alaska Airlines said it was waiting for final revised guidelines from Boeing before it could start inspecting planes and said it was ready to begin as soon as it received required FAA approvals.

Boeing said it was staying in close contact with MAX 9 operators and would help customers address any findings during inspections.

"We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards," the planemaker said. "We regret the impact this has had on our customers and their passengers."

Several industry insiders said airlines have started to hear passengers voice concerns about the safety of the aircraft, even though the MAX 9 in question is only used by a handful of carriers. (Reuters)