United Airlines and Alaska Airlines say they found loose hardware on door plugs on several of their grounded Boeing 737 Max 9 planes, days after a door plug blew off an Alaska Airlines plane while it was in-flight. 

"Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug – for example, bolts that needed additional tightening," United said in a statement to CBS News."These findings will be remedied by our Tech Ops team to safely return the aircraft to service."

Alaska Airlines said in a statement Monday night that, "As our maintenance technicians began preparing our 737-9 MAX fleet for inspections, they accessed the area in question. Initial reports from our technicians indicate some loose hardware was visible on some aircraft. When we are able to proceed with the formal inspection process, all aircraft will be thoroughly inspected in accordance with detailed instructions provided by the FAA in consultation with Boeing."

United has 79 Boeing 737 MAX 9 planes. It didn't say how many had loose bolts. Alaska owns 64 MAX 9s.

• Here's what to know about the Boeing 737 Max 9, the jet that suffered an inflight blowout

United said the aircraft with loose bolts are of various ages, and it doesn't appear the affected planes were part of a group that came off the production line around the same time as the one involved in Friday's incident.

Separately, National Transportation Safety Board officials said in a media briefing Monday night that four bolts that were helping to hold the blown out plug in place are unaccounted for. Investigators don't know if they were ever there or broke or were sucked out of the plane. Further testing will be needed to try to find out.

Friday's incident prompted the FAA to ground all of the types of Boeing 737 Max 9s involved in the incident until the agency is "satisfied that they are safe," an FAA spokesperson said in a statement Sunday.  

Hundreds of flights have been canceled by both carriers since the blowout.

"As operators conduct the required inspections, we are staying in close contact with them and will help address any and all findings," Boeing said in a statement Monday evening. "We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards. We regret the impact this has had on our customers and their passengers."

Alaska and United are the only two U.S. passenger carriers that use Max 9s. The companies operate nearly two-thirds of the 215 Max 9 aircraft in service around the world, according to aviation analytics firm Cirium.

— Kris Van Cleave contributed reporting.