New York, New York - Boeing came under renewed pressure Tuesday as the head of Alaska Airlines said inspections carried out after a dangerous incident had found many loose bolts on its 737 MAX 9 aircraft.

The Federal Aviation Administration grounded 171 Boeing 737 MAX 9 aircraft following the incident on January 5, when a panel blew off the Alaska Airlines plane mid-flight, leaving a hole in the fuselage and forcing an emergency landing.

The company's chief executive, Ben Minicucci, told NBC News Tuesday that an in-house inspection of its Boeing 737 Max 9 fleet carried out after the incident had found that "many" of these aircraft had loose bolts.

"I'm more than frustrated and disappointed. I am angry," he said.

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"This happened to Alaska Airlines. It happened to our guests and happened to our people," he added, calling on Boeing to "improve their quality programs in-house."

Earlier Tuesday, United Airlines cast doubt on future orders and deliveries of Boeing 737s in the wake of the near-disaster, in which no people died or were seriously injured.

In a conference call, its chief financial officer Michael Leskinen said 31 out of the 107 aircraft United expects to take delivery of this year are Boeing 737 Max 9 aircraft.

"It is unrealistic at this time to believe all of those aircraft will deliver as currently planned," he said.

Alongside its MAX 9 orders, United also has 277 as-yet-uncertified Boeing 737 MAX 10 aircraft on order through the rest of the 2020s and options to purchase 200 more aircraft, Leskinen told the conference call.

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"We also expect a reduction in orders and deliveries from Boeing in 2025," he said, noting that orders of 737 MAX 10s would also likely be affected.

The MAX grounding was "the kind of straw that broke the camel's back with believing that the MAX 10 will deliver on the schedule we had hoped for," he said.

"So we are working through an alternate plan," he added.

United's shares finished up 5.3% on Tuesday, the first trading day after Monday's strong fourth-quarter results were published.

Boeing's shares fell 1.6%, and slipped further into the red in after-hours trading.