BIRMINGHAM, Alabama (REUTERS) - An initial investigation into the wreckage of the UPS cargo jet that crashed on Wednesday on approach to Birmingham, Alabama's airport has found no evidence of pre-impact fire or engine failure, the National Transportation Safety Board (NTSB) said on Thursday.

The downed plane's cockpit voice and flight data recorders were retrieved on Thursday.

The so-called black boxes will be sent to the NTSB headquarters in Washington for evaluation of possible clues about the cause of the fiery crash of the United Parcel Service Inc aircraft in which two pilots were killed, said NTSB spokesman Eric Weiss.