NIAGARA FALLS, Ont. — The Rainbow Bridge connecting the United States and Canada was closed Sunday evening as crews dealt with an apparent release of hazardous material, officials said.

The bridge from Niagara Falls, N.Y., to Niagara Falls, Ont., closed about 5:45 p.m., according to emergency dispatchers.

Niagara firefighters were first at the scene, soon followed by hazardous material cleanup teams, a dispatcher told the Hamilton Spectator on Sunday night.

Four people were decontaminated as a precautionary measure -- showered down and put into special suits -- before they were taken to Niagara Memorial Medical Center for a checkup.

Emergency workers were checking 34 others, the dispatcher told the Spectator.

The bridge remained closed hours later, with firefighters, FBI agents and hazardous materials teams on the scene.

Barry Morrissey, spokesman for U.S. Customs and Border Protection, said some officers working on the bridge had throat and nose irritation, though none required hospitalization.

The officers were apparently looking at cash that someone had hidden in their vehicle, said FBI spokesman Paul Moskal.

A substance mixed with the money caused the irritation, Moskal said.

"It doesn't seem to be anything associated with terrorism,'' he said.

Traffic was rerouted to other spans across the Niagara River, including Buffalo's Peace Bridge and the Lewiston-Queenston Bridge.