NORTHWOOD, Iowa — The cause of Thursday's explosion at Northwood AG Products fertilizer storage that contains sulfuric acid is undetermined, according to Worth County Emergency Management Director Ray Huftalin.

The explosion, which forced the evacuation of Northwood's approximately 2,000 residents for most of the day, occurred around 7:30 a.m. Thursday.

Residents were given the all-clear about 4 p.m. U.S. Highway 65 was re-opened at approximately 3:15 p.m. Thursday.

Considering the number of people involved, the evacuation went well, Huftalin said.

Four Northwood residents were treated at Mercy Medical Center-North Iowa Thursday morning following the fire.

The hospital was put on alert for the possibility of more people needing treatment.

Kensett Mayor Tom Dakin breathed a sign of relief following Thursday's emergency.

"The CERT (Community Emergency Response Team) crews and the emergency management groups are just picking up the last of their stuff and pulling out of town if they can get out of town," Dakin said.

"We've been trying to get all the Northwood people out of here. We've been setting car parades to follow an Iowa DOT plow back up to Northwood."

Dakin was proud of how everyone stepped forward to help.

"We did excellent. We did absolutely excellent. I don't think it could have gone any smoother."

The mayor said emergency drills helped get them ready for the real thing.

"We train for these types of events and or course they are spontaneous so you adhere to the plan as best you can and everything went pretty smooth today. It's not often that you evacuate a town."

Steve O'Neil, emergency management director for Cerro Gordo and Franklin counties, assisted with the shelters for Northwood evacuees.

"Like any emergency, it starts off chaotic. And all we try to do is start taking control of the chaos and managing it. We had a lot of excellent cooperation from different agencies," O'Neil said. He commended the volunteers, the people of Kensett and the Northwood residents.

No additional patients had been admitted as of late afternoon Thursday, a Mercy spokeswoman said.

The residents getting hospital treatment came in on their own, a hospital spokeswoman said.

General symptoms of exposure to sulfuric acid are vomiting, nausea and respiratory problems. The spokeswoman said anyone experiencing symptoms should go to the main entrance of the emergency room. A medical team would be there to assist them.