The safety body said its preliminary investigation had found that the plane had been nose-up when it was just 32 feet above ground but was pitched down by three degrees at touchdown.
As a result, the Boeing 737-700's front landing gear, which collapsed on landing, hit the ground first. Ten passengers were wounded in the incident.
The NTSB did not say if the nose-down angle of the plane was responsible for the landing gear collapsing or if it should have been capable of coping with the shock.
The investigation is continuing, with conversations recorded on the cockpit voice recorder due to be transcribed and analyzed on Friday. They may indicate if pilot error was a factor in the accident.
Traumatized passengers recounted after the accident how they had heard a loud bang on touchdown, which was followed by sparks flying as the nose of the plane scraped along the runway for 19 seconds before it came to a halt.
All 150 people on board flight 345 from Nashville were evacuated via emergency slides and 10 were subsequently treated for various injuries.
