One eyewitness reported that the jet flew by with little to no noise, suggesting all engines were inoperative. The most usual cause of engine failure is fuel exhaustion or fuel starvation. A post crash fire, which was also reported tends to eliminate fuel exhaustion as a cause, leaving fuel starvation. Fuel starvation can have a number of causes from contamination, fuel system malfunctions, or poor fuel management. With no reports of distress calls (as of yet) from the cockpit , it would seem the malfunction or condition went unnoticed until it was too late.
Eyewitnesses in cases like this, who have no expertise in aviation matters are notoriously unreliable.
edit -aoy