Boeing admits errors for the first time after the emergency landing of a 737 MAX due to a cabin wall that broke out during flight. The aircraft manufacturer made a mistake and the company will work with regulators to ensure that something like this “can never happen again,” said CEO Dave Calhoun on Tuesday.

The two American airlines Alaska Airlines and United Airlines, which use these aircraft, discovered loose parts on other aircraft of the affected type after the emergency landing. This has raised new doubts about the design and certification process of Boeing’s best-selling aircraft family, which could lead to a repeat of such an incident.

Boeing has instructed its factories and those of its suppliers to ensure such problems are addressed and to conduct more comprehensive checks on systems and processes, people familiar with the matter told Reuters. The accident shocked Boeing customers and also shook CEO Calhoun “to the core,” the insiders quoted the company boss as saying. The company will “ensure that every next plane that takes off is safe,” Calhoun promised employees.

According to insiders, Boeing told employees that the loose screws found were being treated as a “quality control issue” and that reviews were underway at both Boeing and supplier Spirit Aerosystems.

The US Federal Aviation Administration (FAA) grounded 171 aircraft following the incident, leading to numerous flight cancellations by Alaska and United. The panel, which was removed on Alaska Air Flight 1282 at an altitude of 4,900 meters, replaces an optional emergency exit door on 737 MAX 9 aircraft, which airlines use with denser seating. On an Alaska Airlines flight on Friday, the part suddenly tore off shortly after takeoff while climbing.

During initial investigations, Alaska Airlines and the US airline United Airlines discovered problems with other aircraft of the type, such as loose screws.

Because of the incident, United has to cancel around 225 daily flights and Alaska has to cancel 109 flights from its schedule. Boeing shares fell 1.4 percent on Tuesday.