The investigation is under way to explain a “plug” door that blew out shortly after takeoff of a Boeing 737 Max 9 in Oregon on Friday. On Sunday, investigators found what they believe will be a key piece of evidence in that explanation—the plug door itself, discovered in a yard in Portland.

The Federal Aviation Administration has temporarily grounded all Boeing 737 Max 9s until the planes undergo an inspection of their plug doors. This morning, NPR and other media outlets reported three previous flights—Dec. 7 of last year, Jan. 3, and Jan. 4—of “the same plane” experienced an auto pressurization fail light. That reporting quoted NTSB Chairperson Jennifer Homendy initially saying that “the light coming on is ‘very benign’ and it was tested by maintenance crews and reset.” She added, “We don’t know that there was any correlation of the two. It could be entirely separate.”

Alaska Airlines had already restricted the plane from flying over water and requested maintenance crews examine the light, but that request “had not been fulfilled before the plug came off,” according to the reporting.

Again, investigators are placing a good deal of confidence in the examination of the found plug door for answers as to whether this was a result of design, manufacturing, installation, or maintenance of the plug door on these aircrafts.

The incident left the 171 passengers shaken, but unharmed, and they and the six crew members returned safely to the airport in Portland, OR.