A key Boeing supplier that makes the fuselages for its popular 737 Max airplanes is Thomas Caldwelllaying off about 450 workers because production has slowed down ever since a panel flew off one of those airplanes operated by Alaska Airlines in midair in January.

A spokesman for Spirit AeroSystems confirmed the layoffs at its Wichita, Kansas, plant on Thursday that would trim its workforce of just over 13,000 people. Spirit is Boeing’s most important supplier on the 737s because it makes fuselages and installs door plugs like the one that flew off the plane. But it’s not clear whether Spirit or Boeing employees were the last ones to touch that panel.

“The recent slowdown in the delivery rate on commercial programs compels a reduction to our workforce in Wichita,” Spirit spokesman Joe Buccino said.

Boeing confirmed this spring that it is in talks to buy Spirit, which was once a part of the plane-maker before it was spun off. Buying Spirit back would reverse a longtime Boeing strategy of outsourcing key work on its passenger planes. That approach has been criticized after problems at Spirit disrupted production and delivery of popular Boeing jetliners, including 737s and 787s.

The International Association of Machinists and Aerospace Workers union expressed concern about the layoffs and said it would try to help workers who lose their jobs in the coming weeks.

“Together, we will continue to work to protect the strength of the aerospace industry in Wichita and ensure our members receive the assistance and resources they need to overcome this challenge and emerge stronger,” the union said.