The Leo Frigo Bridge was closed Wednesday after a dip was found in span that carries traffic on Interstate 43 across the river, WLUK-TV, Green Bay, reported.
State Transportation Department Secretary Mark Gottlieb says the investigation into the sagging portion was just beginning.
Transportation officials said previous inspections didn't indicate any problems.
"I want to reiterate the bridge is not in any danger of collapse," Gottlieb said during a news conference Wednesday.
Transportation officials said the repair could take months, possibly a year, KLUK said.
Early-morning travelers first detected a portion of the bridge sagging on the east side of the structure, WLUK said. Authorities say a 400-foot section of bridge was affected.
"That bridge is sagging in the center. I came over with a tractor trailer and she jumped that, the wheels came off. There's something that's not quite right," said one caller.
The bridge rests on dozens of concrete reinforced piers, WLUK said. The piers are part of pilings dug very deep into the ground to maintain their structure and are supposed to be immobile, but one moved about three feet into the ground.