The panel that blew off a Boeing Co. jet during an Alaska Airlines flight last month showed signs that it had moved on flights prior to the accident, a US senator briefed on the investigation said Thursday. 

Senator Maria Cantwell, the Democrat who heads the Commerce Committee, said National Transportation Safety Board investigators found marks on the panel indicating it had shifted prior to breaking loose on a Jan. 5 flight over Portland. Cantwell, who is from Washington where the Max 9 jet was built, was briefed by NTSB earlier this week. 

Her comments shed new light on the dramatic accident, which has become the biggest crisis for Boeing since its entire fleet of Max jets was grounded globally in 2019 following two deadly crashes. The NTSB said Tuesday in a preliminary report that the panel, known as a door plug, appears to have left the Boeing factory without four bolts designed to hold it in place.

Cantwell said the evidence raises questions about whether Alaska Air’s maintenance staff might have detected the missing bolts sooner.

“What happened when people are inspecting the planes?,” Cantwell told reporters. “How come that wasn’t detected in the process?”

NTSB spokesman Eric Weiss declined to comment on Cantwell’s remarks, saying only that it would be part of the probe. In the days after the accident, the NTSB said the Alaska jet had experienced repeated issues with its pressurization system but there were no indications they were related.

Boeing and Alaska didn’t immediately respond to requests for comment.