United Airlines found loose bolts on the door plugs on several of its grounded 737 Max 9 planes days after a door plug blew out of 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.”

United Airlines, which has 79 Boeing 737 MAX 9 planes, did not confirm how many planes had loose bolts. 

United said it was working on returning the aircraft into service as soon as possible but expected “significant cancellations” on Tuesday. It has already canceled 200 MAX 9 flights after an Alaska Airlines Boeing 737 Max 9 plane suffered a mid-air incident Friday evening that required an emergency landing.

“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.”

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. 

Alaska and United are the only two U.S. passenger airlines that use Max 9 aircraft. The companies operate nearly two-thirds of the 215 Max 9 aircraft in service around the world, according to aviation analytics firm Cirium. Alaska owns 64 MAX 9s.