WEATHERFORD, Okla. (AP) — A leaking tanker truck in a small Oklahoma city spewed a hazardous gas in a hotel parking lot, forcing hundreds of residents to evacuate and hospitalizing several dozen people, authorities said Thursday.

The anhydrous ammonia gas release happened shortly before 10 p.m. Wednesday. People in the immediate area were in respiratory distress, city officials in Weatherford said at a news conference. Most patients were taken to a nearby hospital, but some, including four in critical condition, were taken to other hospitals for further treatment, Police Chief Angelo Orefice said.

At least 500 to 600 people were in a shelter early Thursday, but others were told to shelter in place, authorities said. That order was lifted later Thursday morning. Several nursing homes were evacuated and schools were closed for the day.

The truck’s driver had parked it behind the Holiday Inn to get a room there for the evening, the police chief said, adding that the cleanup could take several days.

“We pretty much got a lot of of this stuff diluted right now,” and were working with environmental officials, he said.

Krystal Blackwell, who was evacuated, said emergency officials were wearing gas masks and knocking on doors.

“It was a little crazy to wake up to,” Blackwell told KWTV-TV. “I really thought it was a kind of dream.” She said she was sitting in her car, still wearing her pajamas.

Authorities said the air quality was being monitored and that the tanker truck was no longer spewing gas. A number of agencies were on hand to assist, including hazmat crews and an Oklahoma National Guard unit that supports first responders during emergencies.

Right now, the cause of the leak seems to be a mechanical failure on a valve or a faulty seal, Orefice said.

Industrial gas distributor Airgas said it was working with local authorities to respond to the chemical spill.

“Anhydrous ammonia warrants cautious handling and management,” the Radnor, Pennsylvania-based company said in a statement, urging people in Weatherford to follow instructions from authorities in the area.

Anhydrous ammonia is used as a fertilizer to help provide nitrogen for corn and wheat plants. If a person touches it when it is in gas or liquid form, they could be burned. Last week, an anhydrous ammonia leak caused by an explosion at a plant north of Yazoo City, Mississippi, prompted evacuations for nearby residents and sent a plume of yellowish smoke rising above the facility.

Weatherford is home to about 12,000 people and is about 70 miles (113 kilometers) west of Oklahoma City.