As it was a test flight, the incident has not been logged by the NTSB :
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Therefore, no "official" report has been established by an independent body.
"Common" knowledge is that some FOD made its way in the aft electric panel and the whole thing burned down !
Flightblogger published a quite comprehensive summary of that incident
No split over similar-looking wingtips - FlightBlogger - Aviation News, Commentary and Analysis