Originally Posted by The linked article

Unprecedented violence on the opening night of the Wisconsin State Fair by rampaging youths prompted extraordinary measures Friday: The head of the fair implemented new rules to keep unattended teens off the grounds at night, and Gov. Scott Walker ordered the State Patrol to help keep order.
[...]
The trouble at the fair started around 7 p.m. Thursday in the midway area, where amusement rides are located, when fights broke out among black youths, said Tom Struebing, chief of the State Fair Police. Those fights did not appear to be racially motivated.

Then around the closing time of 11 p.m., witnesses told the Journal Sentinel, dozens to hundreds of black youths attacked white people as they left the fair, punching and kicking people and shaking and pounding on their vehicles.