United Airlines (UAL) and Alaska Airlines (ALK) discovered loose bolts on early inspections of Boeing's (BA) 737 Max 9 jets as the aircraft remains grounded following a mid-air incident.

Boeing must provide revised instructions to operators for inspections and maintenance and the jets will remain grounded until aviation regulators determine the aircraft can safely return to operation, the Federal Aviation Administration (FAA) said Tuesday.

Yahoo Finance Legal Reporter Alexis Keenan discusses the finding of the initial inspections and the potential legal fallout.

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Editor's note: This article was written by Luke Carberry Mogan.

Video Transcript

BRAD SMITH: Shares of Boeing, United, and Alaska Airlines giving us a mixed picture in the premarket trading, after the airlines discovered loose parts on their Boeing 737 MAX 9 jets, as they inspect the aircraft days after an Alaska flight left crew and passengers terrified when a door plug blew out at 16,000 feet in the air. Alexis Keenan has the latest on this. Hey, Alexis.

ALEXIS KEENAN: Guys yeah, the NTSB, in a press conference last night, said that they are aware of these reports of loose parts from Alaska Air, and United Airlines, inspecting their Jets after this incident on Friday. But they didn't offer any detail on exactly what those reports say. Now United Airlines, for its part, it has 79 of these MAX 9 jets. They said that under these preliminary investigations, they found instances that appear to relate to installation issues on the door plug. For example, they said bolts that needed additional tightening. Alaska, which has 65 of these jets, they called it loose hardware that was visible on some 737 MAX 9 planes.

But the NTSB yesterday in that press conference, they did disclose some important details about their investigation. They said what broke is what you saw there briefly on the screen, a diagram that has been put out by Boeing of this door. Now there are some fittings, they're called stop fittings. And what they do is, they make sure that door stays in place and can withstand the internal pressure of the aircraft as it goes up into the air.

Now what happened they said is, that some of these bolts were missing when they went and retrieved this door. The NTSB said they don't know if those bolts were ever there, or whether they were blown out when the door was lost on that decompression. Also the NTSB saying that the cockpit door that blew open during this incident, they said that the pilots didn't even know that this jet was designed to do that, and that it's kind of like "Groundhog Day" I feel like here, after having covered the MAX 8 issues in the past with Lion and Ethiopian Air. They said that now Boeing will need to update its manuals so that the pilots are properly informed about the operation of the aircraft. Guys--