KINDRED, N.D. (AP) _ A thick cloud of anhydrous ammonia leaked from a grain elevator Thursday, injuring four people, officials said.

The Cass County Sheriff's office ordered the town of 570 people evacuated after the leak happened at about 7 p.m., but later said people shouldn't leave their homes because the cloud was so thick.

''It's like a cloud, so white, and it's moving very slow, just hovering, because there's no wind,'' said Mary Maurseth, who runs a cafe about two miles south of town. ''It's just going down the streets. It's awful.''

The leak was plugged about an hour after it was discovered, and authorities said residents who fled could return about 9 p.m.

One person was hospitalized in serious condition and two others were in satisfactory condition, Deputy DuWayne Nitschke said. Another person was treated at a hospital and released, officials said.

The leak occurred at the main tank of the Farmers Elevator, said sheriff's dispatcher Kim Stewart. Deputy Lonnie Bjorson said the ammonia spilled while being unloaded at the elevator.

Ron Affeldt, the state's emergency management director, said anhydrous ammonia is used as a fertilizer and is very toxic. ''If you breathe it in and it touches a wet membrane, like your lung, it can burn it. It can burn your eyes,'' he said.

Kindred is about 20 miles southwest of Fargo.