United Airlines CEO Scott Kirby is seeking to reassure passengers after a string of safety incidents raised concerns.

In a letter sent out early Monday to United customers, he said the incidents, though each unrelated, had caught the company’s attention and “sharpened [its] focus.”

“Our team is reviewing the details of each case to understand what happened and using those insights to inform our safety training and procedures across all employee groups,” Kirby wrote.

Within just the past two weeks, United planes were involved in at least four separate incidents, including two that required them to make emergency landings; a runway roll-off and a lost tire.

Kirby said United was already in the process of adding an additional day of in-person training for all pilots starting in May, a more centralized curriculum for maintenance operators, and more resources focused on supplier network management.

United is one of Boeing’s largest customers. The manufacturer remains under multiple federal investigations in the wake of a January incident that saw a door panel blowout midair, forcing an emergency landing. The incidents, among others, have also raised questions about general flight safety, about which some experts have urged customers not to worry.

A United spokesperson did not immediately respond to a request for comment about the carrier’s future relationship with Boeing.

However, last week, Kirby said his airline was putting its orders for Boeing Max 10s on hold and would now weigh whether to order more Max 9s or switch to a plane made by Boeing’s rival, Airbus.