Boeing’s door plug incident is under criminal investigation.

The US Department of Justice’s (DOJ) investigation will consider whether Boeing complied with its 2021 settlement with the DOJ in light of an Alaska Boeing 737 Max that recently lost a chunk of fuselage mid-flight, according to The Wall Street Journal.

Alaska Airlines reportedly called such investigations “normal.” In February, an investigation revealed that four bolts were missing from the door plug when it left Boeing’s factory.