Washington (CNN) -- Investigators so far have failed to turn up any evidence that terrorism was involved in the September 3 crash of a UPS cargo plane in Dubai, a U.S. government official told CNN Tuesday.
White House counterterrorism chief John Brennan told CNN Sunday that the United States was "looking very carefully" at that crash -- in which the two pilots were killed -- to see if it could be related to the recent terror threat involving cargo aircraft.
The Boeing 747 -- operated by UPS -- went down in Dubai, where one of two toner cartridge bombs was recovered just seven weeks later. The other was recovered on a plane in the U.K.
But as investigators in the United States and the United Arab Emirates continue to explore the cause of the crash, they say that at this time, there is no evidence of terrorism.
Government investigators have reviewed the cargo plane's manifest, a U.S. government source said. The official said that there were "a few -- not many, but a few -- packages (on the UPS plane) that originated in Yemen."
"However, they (investigators) have a basic idea of what part of the plane the fire started and these were nowhere near there," said the official, who spoke on condition of anonymity because he was not authorized to discuss the investigation.
Also, officials in the United Arab Emirates have said -- and a U.S. official confirmed -- that the plane's cockpit voice recorder has been examined and nothing on it indicates an explosion. Explosions have distinctive sound signatures, and that would have been recorded on the device, the official said.
The UAE said it has "eliminated the possibility of an onboard explosion, following a detailed onsite investigation of the wreckage." U.S. authorities aren't going that far, saying only that the investigation is continuing. The official noted that a fire caused by an incendiary device, rather than an explosive device, would not have been recorded by the plane's cockpit voice recorder.
About 45 minutes after UPS Flight 6 departed Dubai International Airport for Cologne, Germany, the crew declared an emergency due to smoke in the cockpit. They requested a return to Dubai, but shortly before the plane got to the airport, it crashed.
The cause of the fire is unknown, one official said. One thing investigators are looking at is lithium batteries, which are known to have been on the flight. Even if the lithium batteries were not the source of the fire, they could have made it worse if they were burned.