Regulators in the US have launched an official investigation into the Boeing 737 MAX 9 jet after a window and chunk of fuselage blew out of one in mid-air last week.
The Federal Aviation Administration (FAA) is seeing whether Boeing neglected to check if the panel that flew off the Alaska Airlines passenger plane was safe and manufactured to meet regulatory standards.
Its investigation centres on the plugs used to cover emergency exits that the airline doesn’t use.
The FAA grounded 171 Boeing aircraft, most of which are operated by US carriers Alaska Airlines and United Airlines, until inspections of the jets were completed.
It remains unclear when the planes will get the green light to fly again, in what has been a shaky start to the year for Boeing, the world’s largest aerospace company.
Both Alaska and United said on Monday they had found loose bolts on multiple grounded aircraft during preliminary checks.
They have cancelled hundreds of flights so far, with Alaska saying this equates to 110 to 150 flights per day.
A door plug – which the National Transportation Safety Board (NTSB) says has been recovered – tore off after an Alaska Airlines plane took off from Portland, Oregon, causing depressurisation and forcing pilots to turn back.
The plane, which was flying at 16,000 feet (4,900m) with 171 passengers and six crew on board, landed safely.