Investigators' findings are cranking up pressure on the US aerospace giant after the door of one of their planes tore off mid-flight.
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Two US airlines have discovered loose bolts in their Boeing aircraft.
On Monday, Alaska and United Airlines reported separately that they found loose parts in the panels — or door plugs — of some other Boeing 737 Max 9 jets.
Shortly after these reports federal investigators said they were looking at whether four bolts that were supposed to help hold the panel in place might have been missing after the door of an Alaska Airlines jetliner dramatically fell off last week.
“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,” Chicago-based United said.
"Initial reports from our technicians indicate some loose hardware was visible on some aircraft," added Alaska, having begun examining its Boeing Max 9s.
The findings of investigators and the airlines are ratcheting up pressure on Boeing to address concerns that have grown since the terrifying fuselage blowout Friday night.
A plug covering a spot left for an emergency door tore off the plane as it flew 4,800 meters above the US state of Oregon, eventually landing in a teacher's back garden.
The plane made an emergency landing. No one was seriously injured.
US authorities have since ordered the temporary grounding of Boeing 737 Max aircraft.
Boeing has called an online meeting for all employees on Tuesday to discuss safety.