The Federal Aviation Administration (FAA) said on Thursday it was formally mandating inspections in Boeing 737 MAX airplanes for loose bolts in the rudder control systems after the planemaker recommended them in December.

The FAA said all U.S. airlines had completed the inspections in early January and found no missing or loose rudder bolts. The inspection requirement issued on Thursday fulfills U.S. international continued operational safety obligations.

The issue of bolts and Boeing planes has taken on new significance since the National Transportation Safety Board (NTSB) released its preliminary report this week on the January 5 mid-air emergency of an Alaska Airlines Boeing 737 MAX 9 that lost a cabin panel at 4,900 meters. The agency said evidence suggested the door panel was missing four key bolts.

The NTSB said the panel known as a door plug – fitted into this MAX 9 model in place of an optional exit – could have detached from the plane.

In December, Boeing recommended the loose-bolt inspections after an international operator discovered a rudder bolt with a missing nut while performing routine maintenance, while Boeing discovered an additional undelivered aircraft with a nut that was not properly tightened.

Boeing said on Thursday that since it recommended the inspections in late December, operators have scrutinized more than 1,400 737 MAX airplanes, with only one inspection remaining.

"To date, no other airplane has been found with the condition that initiated the inspection," Boeing said. "Operators who completed the inspection do not have to perform additional examinations and can continue safely flying their airplanes."

Senate Commerce Committee Chair Maria Cantwell told reporters on Thursday that the NTSB told her this week that markings on the recovered 737 MAX 9 door plug suggested that "there was movement" of the door plug on prior flights. "How come that wasn't detected in the process?" Cantwell said.

Alaska and United Airlines both said last month during inspections of MAX 9 jets that they had found loose bolts on numerous airplanes.

The FAA on January 6 ordered 171 MAX 9 jets grounded and then lifted that grounding on January 24 after it mandated extensive inspections of the door plugs. The agency also barred Boeing from boosting the existing 737 MAX production rate, pending improvements in quality control.