An annual Iowa State University (ISU) celebration turned into a riot Tuesday night and police are not quite sure what caused the even to turn violent.

According to the Des Moines Register, Veisha Week is an annual event to celebrate ISU's colleges of veterinary medicine, engineering, industrial science, home economics and agriculture. This year's riot is also not the first time event has gotten out of hand.

The Ames Police Department said people in the crowd tore down two lampposts, four stop signs, flipped cars and threw beer cans and rocks at officers. When one student sustained a serious head injury, emergency responders had trouble making their way through a dense crowd of people.

"That's normal. When you're working crowd management you don't want to take away officers for an arrest," Ames Police Commander Geoff Huff said. "Just because someone wasn't caught last night doesn't mean they're out of the woods."

Police said they were finally able to disperse most of the crowd gathered around the intersection of Welch Avenue and Chamberlain Street by 1:30 a.m. Still, authorities are not quite sure what made the group become volatile.

Huff said he noticed more violence and rowdy behavior after the student injured his head. Before that, he said he just saw people climbing up on signs and posts chanting "Veisha" and "U-S-A."

"That's kind of when things turned a little bit," Huff told IowaStateDaily.com. "A lot of people started to see how serious and how awful this was turning out to be."

That student was apparently hit on the back of the head and was bleeding as he lay on the ground. Huff said that was apparently the most serious injury of the night. He was flown to a Des Moines hospital and there is no new information on his condition.

"I saw a lot of people in the crowd get hit with stuff," he said. "I'm sure there were other injuries, I just haven't heard about them yet. I just can't understand why they do it."

ISU president Steven Leath said in a statement around three a.m. he was not sure whether or not Veisha would continue throughout the week.

Head over to the Huffington Post for more videos and photos from the riot.