What police want to know: is he part of group of 20-to-30 anarchists intent on committing violence?
The vandalism was extensive and expensive for east side businesses. Windows were smashed late Friday night by an apparent anarchist group wearing black ninja costumes.
They did more than $25,000 in damage at five businesses near Prospect and North.
Alert eyewitnesses saw several people leave in a car that lead police to an east side home.
It's where the UW-Milwaukee teaching assistant lives.
He wasn't interested in talking to us about his arrest. But police searched his home and found black clothing including hats and gloves, a sling shot and some material on anarchism.
Police are now searching his computer.
The suspect's lawyer Robin Shellow said, "They have found nothing that distinguished him from tens of thousands of university and college students across the country."
So far, police have only one suspect but they are looking for as many as 20 or more. Are they students? Where they recruited? Is there an anarchist movement on campus? Those are questions police are trying to answer.
UWM student Nick Adams doesn't think there's an anarchist movement on campus but he says if the allegations are true against the teaching assistant it will another black eye for the campus.
"It doesn't make anyone look good at all," said Adams, "We already have enough problems as it is."
The district attorney is still reviewing the case - charges could come Thursday.