[Roccus]

Appears to be the case.

Noticed in the slide show (#4) at this link that the lake is low and shows a drop-off at end of paved ramp.

http://kfor.com/2013/09/17/officials-crews-pull-cars-from-foss-lake-possibly-connected-to-1970-cold-case/

[Blood of Tyrants]

I would guess that they accidentally rolled down the ramp and could not open the doors or windows, panicked and drowned.