Investigators probing why part of a Boeing 737 Max 9 blew open mid-flight earlier this month have not yet determined if bolts were installed on that piece of the aircraft.

National Transportation Safety Board Chairwoman Jennifer Homendy told reporters after a closed-door briefing for lawmakers Wednesday that the investigation into is ongoing. Investigators are determining why a door plug, which is supposed to cover up a space left by a removed emergency exit door in the side of the plane, blew off Alaska Airlines flight 1282 on January 5 and left a gaping hole in the side of the plane.

She said the investigation is not solely focused on the bolts, and her teams are currently collecting extensive records on the assembly of the door plug and its journey from Malaysia, where it was first built, to factories in Wichita, Kansas and Renton, Washington. It was unclear, she said, whether Boeing employees removed the plug from the airplane fuselage when it arrived in Renton from its subcontractor, Spirit Areosystems.

Scientists in the NTSB’s lab are currently scrutinizing the plug but have not yet started disassembling it, she said.

“They have very bright lighting. They’re doing targeted photography,” Homendy said. “They might take some metal shavings and put them under the electron microscope.”

Then next week, NTSB officials will begin pulling the door plug apart to further examine its construction.

Homendy said she and the head of the Federal Aviation Administration, who participated in the Senate Commerce Committee briefing virtually, have been talking once or twice most days.