reporter: the terrifying scene unfolding sunday afternoon just west of milwaukee in waukesha, wisconsin. according to witnesses the suv careening down the street at a high rate of speed. one video showing the car nearly hitting a toddler. the vehicle slamming through barriers and striking parade marchers and spectators. terrified onlookers scattering in all directions. and i saw people on the ground. there s blood and it was really bad. reporter: parents separated from children desperately trying to find them. i ve got 30 down! reporter: one officer opening fire in an effort to stop the vehicle. shots fired, shots fired. reporter: four senior law enforcement officials briefed on the matter tell nbc news that authorities in wisconsin are questioning a person of interest and that he may have a significant criminal history. those sources also say they believe the driver may have been fleeing an earlier knife fight.