(NewsNation) — The activation of warning light that could have indicated a pressurization problem on the Alaska Airlines Boeing 737 Max 9 that lost a door plug shortly after takeoff from Portland does not appear to be correlated with the blowout, the National Transportation Safety Board said Monday night.

The air pressurization warning lights lit up on three different flights Dec. 7, Jan. 3 and Jan. 4 — the day before the accident on Flight 1282. NTSB Chair Jennifer Homendy said at a news conference Monday there is “no indication” the warnings were connected to the loss of the door plug.

Homendy said at a news conference Sunday night crews reported each incident of the warning light illumination. Every time after the reports, the system was tested and reset.

The airlines restricted the aircraft from long flights over water, specifically to Hawaii, so the plane “could return very quickly to an airport” if the warning light reappeared, Homendy said. That was based on Alaska policy, not a federal regulation.

The NTSB said the lost door plug was found Sunday near Portland, Oregon, in the backyard of a Portland teacher identified as Bob. 

The four bolts that prevent the door plug from moving have not been recovered, investigators said Monday. The door plug will be taken back to a NTSB lab in Washington, D.C., and examined to determine if it was ever actually bolted to the fuselage.

When the “explosive decompression” happened, the cockpit door flew open immediately, hit the lavatory door, which caused the lavatory door to become stuck. Homendy said the cockpit door on the Max 9 is designed to open automatically in the event of a depressurization, but flight attendants were not aware of that.

Communication became a serious issue and the flight attendants said it was hard to get information from the flight deck.

The scene was described as “chaos,” “very loud” and “very violent.”

Investigators will not have the benefit of hearing what was going on in the cockpit during the flight. The cockpit’s voice recorder — one of two so-called black boxes — recorded over the flight’s sounds after two hours, Homendy said.

Two cellphones that appeared to have belonged to passengers on Friday’s terrifying flight were found on the ground. One was discovered in a yard, the other on the side of a road. Both were turned over to the NTSB.

The plane made it back to Portland, and none of the 171 passengers and six crew members were seriously injured.

Hours after the incident, the FAA ordered the grounding of 171 of the 218 Max 9s in operation, including all those used by Alaska Airlines and United Airlines, until they can be inspected.

Alaska Airlines and United Airlines canceled 342 flights Monday due to the grounding, CNN reported.

The Associated Press and KOIN contributed to this report.