The Winnebago County Sheriff's Office says the helicopter went down into the river near the Wisconsin Street Bridge in Oshkosh, 94 miles north of Milwaukee after witnesses said it hit the electrical wires.

OSHKOSH, Wis. — A helicopter crashed into the Fox River in eastern Wisconsin Saturday after witnesses said it struck power lines, and authorities said they won't be able to search for those on board until high voltage power lines can be removed from the water.

The Winnebago County Sheriff's Office says the helicopter went down into the river near the Wisconsin Street Bridge in Oshkosh, 94 miles (151 kilometers) north of Milwaukee after witnesses said it hit the electrical wires.

Winnebago County Sheriff's Capt. Lara Vendola-Messer said during a press conference that special equipment was being brought in by Wisconsin Public Service to help remove the lines. It's unknown how many people were onboard or what happened to them.

"Other than it is a helicopter, we don't know where it came from or anything else at this point," Vendola-Messer said.

Gary Lemiesz, of Oshkosh, told The Oshkosh Northwestern that he watched as a dark green helicopter flew east over the Fox River before turning back and flying west. He said it struck the wires and part of the propeller broke off, after which "it hit hard and sunk."

Oshkosh Fire Dept. Assistant Chief John Ziemer said boats did a surface search but did not find anyone in the water.

Officials say the crash happened on a busy weekend in the city. Increased boat traffic for an annual charity poker run — a poker tournament that includes various stops on the water — had to re-routed from the area. In addition, officials for a walleye tournament had to change their weigh-in destination.

It is the second helicopter crash in Wisconsin this year. Three people died in April when a medical helicopter crashed near Hazelhurst, in the northern part of the state.