Checking the maps of the location below, the Model S had to drive to the right to enter the rest area, to the left to enter the truck parking zone, and again to the left to hit the tractor-trailer rear. Considering the damages to the vehicle, it does not seem to have reduced its speed. The picture released by FHP shows no signs of braking before the crash.
In most crashes involving Autopilot, the system only failed to detect obstacles ahead and kept on driving as if they were not there. Although Autopilot can steer, brake, and accelerate the car, this would be the first incident in which it actively steered the vehicle until the wreck stopped it. Full Self-Driving (FSD) can also perform these tasks.
Tesla has not said so far if any of its ADAS systems were engaged in the vehicle. In fact, it has to disclose if it was engaged at all in the trip after NHTSA said that "Autopilot aborted vehicle control less than one second prior to the first impact" in crashes against emergency vehicles. If Autopilot were not involved, the driver would have driven into the rest area and hit the tractor-trailer on purpose, which does not make much sense if she could have done the same on the highway with any other tractor-trailer that was around.
There is no word about who is investigating the crash. The possibility that Autopilot is involved may put NHTSA on board. The National Transportation Safety Board (NTSB) has also been following weird crashes involving Tesla with attention, so it may also want to verify what caused this incident. With the EV maker’s ADAS under scrutiny, ruling out their involvement is mandatory.