I do not know what caused this to happen. There were no warning lights or messages. Prior to this incident the transmission would rev/speed up on its own for a few seconds (I don't know if this is relevant). Salesperson told me about this and said it was nothing to worry about. The vehicle is equipped with a sensor that brakes if you come to close to an object. This sensor has definitely worked in the past. I was backing up down a long driveway, approximately 50 yards.When I took my foot off of the gas pedal, it did not slow down. Instead it sped up and was uncontrollable. I looked down to make sure my foot was on the brake which it was. I then used both feet to push down on the brake but that did not work either. The vehicle did not stop until it ran into a building. If somebody would have been behind me they would have died or been seriously injured. If the building would not have been there, I might have died or been seriously injured due to the vehicle accelerating at a high speed on its own and the brakes failing to stop the vehicle. This incident happened yesterday so it has not been reproduced or confirmed by a dealer or inspected by the manufacturer or insurance. It happened on private property and will not be inspected by the police.