The U.S. National Transportation Safety Board (NTSB) said Tuesday multiple systemic failures led to a mid-air door panel blowout of a Boeing jet last year, faulting Boeing and the Federal Aviation Administration (FAA) for training and oversight failures.

Boeing failed to "provide adequate training, guidance and oversight" to its factory workers, the investigative agency reported in a news release.

In an investigation spanning some 17 months, the NTSB found that four bolts securing the door panel were not in place in a new Boeing 737 MAX 9 jet during its production at Boeing's factory.

The door panel flew off the aircraft of Alaska Airlines in mid-air shortly after the jet took off on Jan. 5 last year from Portland, the U.S. state of Oregon. The crew made an emergency landing that ensured the safety of all people on board.

Boeing's manufacturing and safety oversight, combined with the FAA's ineffective inspections and audits, led to the malfunction, the NTSB noted.

Both Boeing and the FAA have improved training and inspections since the mid-air emergency occurred, but they need to better identify such risks to prevent them from sneaking through again, the NTSB said.