PB
I will describe from memory here an article I read about that fateful event with the car going into the water with Mary Jo Kopechne in it. It might be bullshit, or might not. What was said was that Mary Jo Kopechne was at a party of people associated with the Ted Kennedy campaign. She wasn't feeling well or maybe had been drinking, and got into the back seat of that car to lie down.
Ted Kennedy was in the midst of a major affair with the wife of a very high-up politician, she also was at this party. The two decided to slip away and were driving in the car when it went over the bridge and into the water. They did not know Mary Jo was in the back seat. They climbed out of the water, I don't remember the specifics at this point, but what was significant was how it was described that Kennedy was acting like nothing was amiss when he got back to wherever.
So the takeaway is that he suffered the hit to protect this woman. Whatever the truth is, maybe members of the family knew, maybe Batman and / or Robin know.