FARGO (AP) - Workers building a $25 million hockey arena got to the site Monday and found vandals had started up huge cranes, raised the boom on one of them and bent a beam supporting the roof.

Construction site manager Trevor Speidel said the vandals used a crane to smash a concrete block, shattered windows and threw equipment off the roof of the Urban Plains Center, punching a hole in the concrete below.

The vandalism likely happened late Sunday night or early Monday, Speidel said.

Police said one of the cranes was inside the building, and another was outside. Damage to the building and the cranes was estimated at more than $500,000.

Police Sgt. Jeff Skuza said the cranes did not appear to have been secured by workers.

"There was no evidence of forced entry or hot-wiring," Skuza said. "We're operating under the assumption that the keys were left in them."

An initial report from the building architect indicates the damage will not affect the Oct. 31 opening date, said Todd Berning, president of the Metro Sports Foundation, which runs the UP Center.

"We just have to work through it," Berning said.

It was not clear how the vandals got access to operate the cranes, he said.

The procedure is for workers to take keys out, but the keys were in and the cranes were running when workers arrived Monday morning, Berning said.

Other vandalism at the site included the word "Yeah" written on the side of the building. A generator was among the items thrown off the roof, Berning said.

The builders and Metro Sports Foundation have notified their insurance companies, he said.

The UP Center's 5,000-seat main rink is for Fargo schools and the United States Hockey League's Fargo Force, led by former University of North Dakota hockey coach Dean Blais.

Berning said extra security will be put in place at the site.

"We're just lucky nobody got hurt. We're lucky we can fix it and keep moving," he said.

"They didn't understand the controls (of the cranes)," Speidel said. "If that crane would have been a little bit farther back, they would have tore that whole beam down."

It could have created a domino effect, "and you'd probably have lost half the building," he said.