Alaska Airlines chief Ben Minicucci told NBC that the company had found loose bolts in "many" of its Boeing 737 Max 9s during inspections, as both he and United Airlines' CEO raised concerns about the aircraft maker on Tuesday.

Why it matters: The findings come after one of Alaska Airlines' Boeing 737 Max 9 planes lost its exit door plug during a flight earlier this month — prompting grounded aircrafts, a multi-passenger lawsuit and further questions about the aircraft's safety.

• While Minicucci, who is Alaska Airlines' president and CEO, urged Boeing to "improve their quality programs in-house," United Airlines CEO Scott Kirby said the company is looking at fleet plans that do not include the Boeing 737 Max 10.

What they're saying: "'I'm angry. I'm more than frustrated and disappointed," Minicucci said in an interview with "NBC Nightly News with Lester Holt," airing Tuesday.

• "This happened to Alaska Airlines. It happened to our guests and happened to our people," he said.
• NBC's Tom Costello asked whether Boeing has an issue with "quality control" that extends past just one plane.
• "I think this is the issue that's at question right here, which is what is Boeing going to do differently on their quality program, to make sure that when we get an airplane, it's at the highest degree of excellence and that's what's got to be different going forward," Minicucci replied.

Meanwhile, Kirby called the Max 9 grounding "probably the straw that broke the camel's back for us" during an interview with CNBC on Tuesday.

• "We're gonna at least build a plan that doesn't have the Max 10 in it," Kirby said.
• "Now, we'll hope that Boeing gets it certified at some point, but we're gonna build an alternative plan that just doesn't have the Max 10 in it," he added.

Of note: That plane has not yet received certification from the Federal Aviation Administration.

Separately, Kirby clarified on a conference call that United has removed the the Max 10 from its internal plans, rather than canceled its order, per Bloomberg.

• "Boeing's not going to be able to meet their contractual deliveries on at least many of those airplanes," Kirby added.

The other side: "We have let down our airline customers and are deeply sorry for the significant disruption to them, their employees and their passengers," Stan Deal, president and CEO of Boeing Commercial Airplanes, said in a statement shared with Axios.

• "We are taking action on a comprehensive plan to bring these airplanes safely back to service and to improve our quality and delivery performance. We will follow the lead of the FAA and support our customers every step of the way," Deal said.

What's next: Boeing announced in an internal memo obtained by Reuters and other outlets that its 737 factory in Washington will have a "quality stand down" on Thursday.

• Production and delivery will be paused so employees can participate in "working sessions focused on quality."

Go deeper: FAA advises door plug checks for Boeing 737-900ER jets

Editor's note: This article has been updated with details of the stand downs.