NTSB Chair Jennifer Homendy told a press conference late on Saturday the two seats next to the portion of the fuselage that blew out were unoccupied.
"We are very, very fortunate here that this didn't end up in something more tragic," Homendy said.
Parts of the seat next to the fuselage, including the headrest, were missing.
US aviation officials have ordered all Boeing 737 Max 9 planes to be inspected after the incident. (AP PHOTO)
The Federal Aviation Administration (FAA) on Saturday grounded 171 Boeing 737 MAX 9 jetliners for safety checks after the emergency landing of the plane that had been in service for just eight weeks.
A piece of fuselage tore off the left side of the jet as it climbed following take-off from Portland, Oregon, en route to Ontario, California, forcing pilots to turn back and land safely with all 171 passengers and six crew on board.
Some minor injuries were reported, Homendy said.
The FAA did not rule out further action as a probe began into the apparent structural failure, which left a rectangular hole in an area of fuselage reserved for an optional extra door but which is deactivated on Alaska's aircraft.
Investigators will look at maintenance records, the pressurisation system and the door components, Homendy said.
"We'll go where the investigation takes us," she said, asking for the public's help in recovering the missing door plug believed to be in a suburb west of Portland.
Homendy praised the FAA for swiftly grounding the MAX 9 to "ensure continued safety."
Alaska Air said it had halted flights by 18 of its MAX 9 planes that it had resumed using Saturday after recent in-depth inspections.
The airline said it was in discussions with the FAA "to determine what, if any, further work is required before these aircraft are returned to service".
The FAA could announce inspection requirements as early as Sunday, officials said.
The Boeing 737 MAX 9s fitted with a special door replacement "plug" cannot fly until they are inspected and repaired if necessary, the FAA said.
A section of the fuselage reserved for the optional door had vanished, leaving a neat door-shaped gap.
The extra door is typically installed by low-cost airlines using extra seats that require more paths for evacuation.
However, those doors are permanently "plugged," or deactivated, on jets with fewer seats, including those of Alaska Airlines.
The Alaska Airlines plane involved in the incident had only been in service for eight weeks. (AP PHOTO)
The fuselage for Boeing 737s is made by Kansas-based Spirit AeroSystems, which separated from Boeing in 2005.
Spirit manufactured and installed the particular plug door that suffered the blowout, a source told Reuters on Saturday.
The MAX 9 represents about 220 of the 1400 MAX jets delivered so far and most of them have the deactivated door, meaning they are potentially covered by the order.
Boeing said it supported the FAA decision.
MAX planes were grounded worldwide for 20 months after crashes in Ethiopia and Indonesia, linked to poorly designed cockpit software, killed almost 350 people about five years ago.
Boeing has suffered numerous production issues on the MAX planes in the years since the crashes.
The manufacturer on December 28 urged airlines to inspect all 737 MAX airplanes for a possible loose bolt in the rudder control system.
Flight 1282 had reached just more than 4877m when the blowout happened, according to FlightRadar24.
"We'd like to get down," the pilot told air traffic control, according to a recording posted on liveatc.net.
"We are declaring an emergency."
Australia's air regulator - the Civil Aviation Safety Authority (CASA) - said there were currently no Boeing 737 Max 9s flown by any airlines in Australia.
