According to news reports, it wasn't a door though. It is said it was a window located at a place where a door could be installed as an option. If it's the case

According to news reports, it wasn't a door though. It is said it was a window located at a place where a door could be installed as an option. If it's the case, I suppose there was not specific maintenance on this part of the aircraft. I do agree with you that proper investigation is required before jumping to any conclusion, plenty of factors (external or internal to boeing) can contribute into this type of incident.