This new footage reveals why NTSB investigators are focusing on the left engine in the UPS cargo plane crash in Louisville … it had fallen clear off the aircraft by the time it went wheels up.

Someone’s dashcam captured the horrifying footage, which shows the massive three-engine jet only had its right engine seconds after lifting off … making it impossible for the pilot to get enough power to avoid the tragedy that killed at least 12 people this week.

Investigators announced Wednesday they’d recovered the flight and data recorders from the crash as they try to figure out what caused the disaster. While they already knew the left engine had fallen off the MD-11 jet … they don’t yet know why it separated from the plane.

As we reported, the plane burst into a massive ball of fire Tuesday evening as the 3-person crew was taking off to begin a flight to Hawaii.

The cargo plane was so low to the ground, it was clipping power lines — you can see sparks flying in the video moments before the crash.