Car Accident - Am I At Fault?

[creativeartist20]

I was driving through a 30 mph speed zone about 3 and 1/2 cars behind another car. Out of nowhere, the car ahead of me stepped on the brake completely at an intersection with no stop sign because she claimed that another driver looked like he was about to pass (which he didn't). I tried to stop completely, but didn't make it in time (it was also raining at the time). Her car had only a few minor scratches on her bumper while my car was smashed up from front. She claimed insurance and I'm wondering if I'm going to be at fault? The only witness to this whole accident was my friend that was sitting in the passenger seat.

[Mr. Knowitall]

The driver who rear-ends another car, going the same direction in the same lane of traffic, will almost always be found to be at fault.