It looks like the airplane followed a parabola, maintaining forward velocity while building up vertical velocity, as evidenced by the near constant GPS (=ground) speed in the FR24 data. As Sumwalt stated in the briefing, the aircraft flew wings level but in a steep nose down attitude. Lack of elevator effectiveness springs to mind. In the presumed absence of intent (i.e. suicide) it remains to be seen whether that's a runaway trim, complete loss of elevator control, or loss of elevator/tail surfaces. In level flight, the tail surface provides a net down force, lose some of that surface, and an up moment of the tail ensues resulting in a nose down moment on the pitch axis.