A door plug fell out of an Alaska Airlines flight Friday, NTSB officials said.

By Kevin Shalvey

The focus of the investigation into Friday’s midair emergency on an Alaska Airlines flight is focused on the single aircraft, but could be broadened as the National Transportation Safety Board learns more, board Chief Jennifer Homendy said.

“However, at some point we may need to go broader. But right now we have to figure out how this occurred with this aircraft,” Homendy said Tuesday on ABC News’ “Good Morning America.”

The door plug fell off the plane, a Boeing 737 Max 9, around 5:11 p.m. local time Friday as the aircraft with 171 passengers, including three babies and four unaccompanied minors, had climbed to 16,000 feet after taking off from Portland International Airport, according to the NTSB.

The fittings at the top of the door plug fractured, Homendy said.

“We don’t know if the bolts were loose. We don’t know if bolts were in there fractured or possibly the bolts weren’t there at all,” she said. “We have to determine that back in our laboratory.”

On Monday, United Airlines said it had found loose bolts on its 737 Max 9 fleet during inspections ordered after Friday’s incident involving an Alaska Airlines flight.

The Federal Aviation Administration said Saturday it was temporarily grounding certain Boeing 737 MAX 9 aircraft operated by U.S. airlines or in U.S. territory until they were inspected. The FAA said the pause would affect about 171 planes worldwide.

Homendy said Tuesday she would feel safe flying on a 737 Max 9 now.

“I would feel safe flying right now,” Homendy said. “Our aviation system is the safest in the world.”

ABC News’ Amanda Maile, Sam Sweeney, Bill Hutchinson and Jon Haworth contributed to this story.