The brakes are mechanical, not software based. So there are two things that could have happened here. 1 - driver presses gas instead of brakes and panics, continuing to hold down gas. This is what the blackbox data of the car will show, as it almost always does in these crashes. 2 - the car decided to get stuck on acceleration, somehow, and simultaneously the hydraulic brakes failed at the same moment. This assumes the driver is attempting to apply the brakes the entire time (we can't see brake lights, so unlikely, probably holding down the accelerator instead). We know that if functional the brakes can easily overpower the motors.
So, either 1 driver failure OR both a software, electrical and mechanical failure in multiple different systems all at the same time.
Which one seems the most realistic? Let's wait for more investigation.