The U.S. National Transportation Safety Board (NTSB) is conducting a new round of interviews with Boeing and Federal Aviation Administration (FAA) personnel this week in its probe of the January Alaska Airlines 737 MAX 9 in-flight emergency.

NTSB Chair Jennifer Homendy on Wednesday said investigators were back at the 737 plant in Renton, Washington, this week for more interviews.

"We are looking at other instances where a door plug was opened and closed to make sure that those records are available," Homendy said at a U.S. Senate hearing, noting that investigators want to make sure those other instances were documented.

Last month, Boeing said it believed that required documents detailing the removal of the door plug in the Alaska Airlines plane involved in the emergency were never created.

Homendy said on Wednesday that Boeing and investigators still do not know the personnel who worked on the Alaska Boeing 737 MAX 9 that suffered the emergency. "This work occurred in September. They move a lot of planes through that factory," Homendy said. "The biggest concern is missing records."

At issue is the process not the individuals, she said. "This isn't a gotcha on anybody," Homendy said, adding the NTSB has still been unable to interview the door plug team manager, who has been on sick leave.

The door plug panel blew off the Alaska Airlines flight not long after the plane took off from Portland, Oregon, on January 5.

Homendy said investigators are also looking at the planemaker's safety culture. She said the NTSB could conduct a safety culture survey at Boeing.

Homendy said the planemaker was working well with the NTSB and providing documents sought. Boeing wants to understand what went wrong: "They want to know and they want to fix it," Homendy said.

Following the incident, the FAA grounded the MAX 9 for several weeks, barred Boeing from increasing MAX production, and ordered the company to address systemic quality-control issues within 90 days after an audit found fault with the company's manufacturing processes.

The NTSB said previously that four key bolts were missing from the door plug that blew out.

The Justice Department has opened a criminal investigation into the emergency.

Homendy in March criticized what she called Boeing's lack of cooperation and failure to disclose some documents, including on the door plug opening and closing, as well as the names of 25 workers on the door crew in Renton. After Homendy's comments, Boeing provided the 25 names, and the planemaker said it was cooperating.

The NTSB plans to hold a public investigative hearing into the Alaska Airlines incident on August 6-7.

Homendy said the hearing would include testimony from employees at Boeing and fuselage manufacturer Spirit AeroSystems and others like Alaska Airlines.