@drwho I'm not fully versed on the history, but one of the responses in this thread mentions that separate, earlier issue of the physical problem with the floor mats.
This article mentions and links the NASA/NHTSA investigation. This related article says more:
Neither NHTSA, with its absence of software expertise, nor the NASA Engineering and Safety Center — to which NHTSA turned to study the Toyota problem — were able to pinpoint a software cause for unintended acceleration. Nor were they able to rule out the possibility.
The NASA researchers, who were both on a deadline and not allowed to study Toyota's source code, simply ran out of time, noted Barr.