ABX Air, a contractor for DHL that is also known as Airbourne Express, received a threat against them, although no specific aircraft was mentioned. However, the NTSB also said that initial investigations have revealed no evidence of a bomb or an incendiary device.

The fire at 10 p.m. Saturday broke through the jet's exterior just behind the cockpit. The two-man crew had been preparing to start up the jet's engines for takeoff at the time, but escaped without injury. Both have been interviewed by the NTSB, and the Cockpit Voice Recorder and Flight Data Recorder have both been sent to Washington where they are being analysed at the NTSB's headquarters.

Records kept by the NTSB and the Federal Aviation Authority indicate that the aircraft, which was manufactured in 1987, had had no previous issues. It had transported passengers until 2004, when Israel Aerospace Industries converted the plane for cargo transport. Israel's Ministry of Transport has sent a representative to assist the investigation, and Israel Aerospace Industries will serve in an advisory capacity.

Also assisting with the investigation were the FBI, the Bureau of Alcohol, the Transportation Security Administration, the San Francisco Fire Department and the San Francisco Airport Rescue and Fire Fighting division. Under International Civil Aviation Organization (ICAO) Annex 13 other firms involved with the aircraft, such as Boeing and ABX, will also be party to the investigation.

The NTSB expects the on-scene portion of the investigation to be completed by July 6.

Source: Wikinews
Henry K. Lee "Air cargo firm was threatened before plane caught fire at SFO". San Francisco Chronical, July 4, 2008
"Threat received before US cargo plane caught fire". Aviation Record, July 4, 2008
"Threat made against cargo airline before Saturday fire in plane". Mercury News, July 3, 2008