Investigators search for a Boeing door that was torn off during the flight in the suburbs of Portland, US | the world

U.S. authorities are appealing for the public's help after a door tore off a new Boeing 737 MAX 9 plane near Portland, Oregon, after an explosion ripped it open during the day.

After takeoff from Portland to Ontario, California, a door plug tore off the left wing of an Alaska Airlines jet, forcing the pilots to turn back and land safely with 171 passengers and six crew on board. Only a few people were reported to have sustained minor injuries.

A door installed on some planes instead of an extra emergency exit may have fallen somewhere in Portland's western suburbs, but has yet to be found.

“I imagine it was an absolutely terrifying event. We don't often talk about the psychological consequences, but I believe that happened here,” National Transportation Safety Board (NTSB) Chairman Jennifer Homandy told reporters Saturday. It is too early to say what caused the incident.

Homandy said the two seats next to the section of the fuselage that exploded were unoccupied. An independent US agency launched an investigation.

“We're very fortunate that it didn't lead to something more tragic,” Homandy said. Parts of the seat near the fuselage are missing, including the headrest.

An additional exit door is usually installed by low-cost airlines that use more seats and require additional exit routes. However, these doors are blocked on jets with fewer seats. For passengers, the area looks like a regular window seat.

The crash has put Boeing under renewed scrutiny as it awaits certification for the smaller MAX 7 and larger MAX 10, which it needs to compete with the more successful Airbus model.

The US Federal Aviation Administration (FAA) on Saturday grounded 171 Boeing jets that had the same panel installed following the emergency landing of an Alaska Airlines plane that had been in service for eight weeks. The move is expected to disrupt the flight schedules of some airlines for a few days.

Most of the aircraft of this model used in the United States are operated by United Airlines and Alaska Airlines. Turkish Airlines, Panama's Copa Airlines and Aeromexico grounded flights for inspections.