NTSB chair Jennifer Homendy told reporters after a Senate hearing the aeroplane had been in for maintenance the night before the incident in which it lost the engine cowling.

“It’s a maintenance issue,” Homendy said, adding the board opted not to open a formal investigation. The NTSB had sent a structural engineer who lives in Denver to look at the plane but was satisfied with its understanding of the incident.

“Southwest is addressing it,” she added.

Prior incidents involving engine cowling have been attributed to failure to latch the fan cowl doors. Homendy said in the Southwest incident “there may be some issues with how they latch to see whether they can tell if they’re locked or not. So they’re going through those procedures now.”

The Federal Aviation Administration, which is investigating, did not immediately comment on Wednesday. Southwest said it would “defer to the FAA due to the ongoing investigation into this incident.”

No one was injured and Southwest Flight 3695 returned safely to Denver International Airport around 8.15am on Sunday and was towed to the gate after losing the engine cowling.

The Boeing aircraft bound for Houston Hobby airport with 135 passengers and six crew members aboard climbed to about 3,140m before returning 25 minutes after takeoff.

The plane entered service in June 2015, according to FAA records. Boeing referred questions to Southwest.

The 737-800 is in the prior generation of the best-selling 737 known as the 737 NG, which in turn was replaced by the 737 MAX.

Boeing has come under intense criticism since a door plug panel tore off a new Alaska Airlines 737 MAX 9 jet at 4876.8m on Jan 5.

In the aftermath of that incident, the FAA grounded the MAX 9 for several weeks, barred Boeing from increasing the MAX production rate and ordered it to develop a comprehensive plan to address “systemic quality-control issues” within 90 days.

The FAA is investigating several other recent engine issues on Southwest’s fleet of Boeing planes.