 The National Transportation Safety Board (NTSB) investigated the accident.Although the tail of the Delta DC-9 was several feet from the runway, the NTSB determined that DAL 379's engines were still operating at idle when Flight 159 attempted to take off, and DAL 379's idling jet engines were directing hot jet exhaust over the runway.The NTSB determined that neither Federal Aviation Administration (FAA) regulations nor the Terminal Air Traffic Control Procedures Manual defined the phrase "clear of the runway," and found that the pilots of each plane and the air traffic controller each had their own slightly different definition of the term.