Alaska Airlines and United Airlines both said they discovered loose parts on several grounded Boeing 737 MAX 9 jets just a few days after a near-disastrous event occurred on an Alaska Airlines flight.

The disclosure came shortly before National Transportation Safety Board (NTSB) investigators said that a door plug on Alaska Airlines flight 1282 somehow moved off a set of stops designed to keep it attached to the plane, causing a dangerous blowout to occur at around 16,000 feet January 5.

According to The Wall Street Journal, the four bolts designed to keep the plug from moving off the spots were missing, said NTSB investigators, who added that they are working to determine if the equipment was ever installed in the first place.

United Airlines said that its inspections “found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening.”

Meanwhile, Alaska Airlines said that the original reports from technicians preparing its fleet for inspections accessed the plug area and found loose hardware was visible on some planes.

“Any findings will be fully addressed in a matter that satisfies our safety standards and FAA compliance,” Alaska Airlines said.

The NTSB said January 8 that it will focus on determining what caused the door plug to move out of position on Alaska Airlines flight 1282 before deciding whether or not to broaden its probe of Boeing 737 MAX jets.

“We cannot tell you at this time how or why,” said NTSB chair Jennifer Homendy. “We do not have that information. We will have that information. It’s going to take time.”

The FAA issued an emergency order January 6 that temporarily grounded numerous 737 MAX 9 jets, resulting in hundreds of flight cancellations.