A&E's Investigative Reports did a story on this, a two hour episode, in fact. Their reconstruction established that Kopechne would have to have been driving the car to receive the injuries to her side that resulted. I can't remember every detail, but the only scenario that fit the facts, once they established she was driving the car, was that Kennedy got out of the car to avoid being caught drunk at the wheel with a young blonde in the car. Some police had rousted them from a "lover's lane"-type spot earlier. Kennedy gets out of the car, tells her to drive car to place X where he will meet her, and she drives off the bridge in the dark--the car was large, huge compared to her own VW, and she was probably under the influence. Hours and hours later, Teddy gets a phone call about them pulling a car out of Chappaquidick, and he has to make up a story...
I'm not sure that's what happened, but if true it explains a lot, like why she didn't just swim out after him when he escaped: she couldn't, because no one escaped from that car, and Kennedy didn't know she was there...
Their reconstruction established that Kopechne would have to have been driving the car to receive the injuries to her side that resulted...yes, I think this is the latest, most likely theory of what happened that night - apparently Kennedy and Kopeckne were on the way down the dirt road to the beach for a makeout session when they were spotted turning off of the main road by a local policeman...Kennedy was possbily afraid they were going to be followed, so he left the car and hid in some bushes to let Kopeckne drive on down the road, which wasn't familiar to her...she didn't realize there was a bridge on the road and was going too fast to navigate it when she started over....
There were so many pieces that didn't fit in Kennedy's story that an honest prosecutor probably could have nailed him for obstruction of justice, if nothing else - for instance, he actually claimed that he swam back from the island to the mainland in the middle of the night, across a channel with fierce currents, in a backbrace...and then there were the questions of whether Kopechne was pregnant at the time, since her body was removed from the island for autopsy elsewhere even though the law required the autopsy be done locally if someone died there.....
He's pure scum.....