The overhead video from the drone shows a red minivan plowing through the crowd. It definitely looks like it's hitting people. The end of that video shows the minivan in the intersection, with the light silver car with the black top behind.
Who were the drivers of those two cars?
If two other cars drove through the mob just before Field's Challenger did, that seems kinda important.
The answer to your second question: The police skedaddled. They felt the situation was too dangerous.
The white car and red minivan were pushed into the intersection by the ram-car.
https://twitter.com/Breaking911/status/896451158319013893