Industry publication Air Current reported that United found discrepant bolts on other parts on at least five panels that were being inspected following the accident. The U.S. Federal Aviation Administration (FAA) and Boeing declined to comment. "Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug – for example, bolts that needed additional tightening. These findings will be remedied by our Tech Ops team to safely return the aircraft to service," United said in a statement.

Boeingand U.S. regulators gave the go-ahead on Monday for airlines to inspect grounded jets. The inspections of 171 MAX planes mostly operated by U.S. carriers Alaska and United Airlines may take several days, forcing the cancellation of numerous flights.

Boeing shares sank 8% Monday as the company reckons with the aftermath of the surprising panel emergency. It was the latest setback for Boeing following a series of production delays that have hampered its recovery from a lengthy 737 MAX safety grounding in 2019.

The FAA ordered the temporary grounding for checks of 171 MAX 9 jets installed with the 60-pound (27 kg) panel that detached from a full Alaska Airlines Flight on Friday and crashed into a Portland suburb without harm.