Both United Airlines and Alaska Air said in the days after the explosion that they found dismantled parts on several grounded MAX 9 planes.
The plug is manufactured by Spirit AeroSystems, a subsidiary of Boeing that spun off from its parent company in 2005. The production process involves work at its Wichita, Kansas, facility before the fuselages are transported to Boeing's factory in Renton, Washington.
The FAA has taken a tougher stance than in the past toward Boeing. In late January, Boeing was prevented from expanding production of its 737 MAX aircraft due to quality problems. This means that it can continue to produce MAX aircraft at its current rate, but it cannot increase that rate.
“I certainly agree that the current system is not working, because it does not provide safe aircraft,” FAA Administrator Mike Whitaker told politicians this week. “So we have to make changes to that.”