A UPS plane that crashed in a fireball in Kentucky in November, killing 15 people, had a structural flaw identified by Boeing on similar planes 15 years ago, according to investigators.

BBC News reports that the MD-11F freighter crashed after one of its engines separated from the wing as it was preparing to take off from Louisville, Kentucky.

In an undated update report, the U.S. National Transportation Safety Board (NTSB) determined that cracks found in the engine mounting assembly had previously occurred on several other aircraft.

The report also found that Boeing had previously found failures of the same part on four occasions and, in 2011, had sent a "service letter" to operators warning them of its findings. This type of document, which is not legally binding, is used by the planemaker to alert operators about important safety or maintenance information.

At that time Boeing concluded that the issue "would not result in a safety of flight condition."

In a statement, Boeing said: "We continue to support the investigation led by the NTSB. Our deepest condolences go out to the families who lost loved ones and our thoughts remain with all those affected."

The NTSB's investigation continues, and it has not yet issued any firm conclusions about the cause of the accident. It is unlikely to do so until it publishes a final report.