Three chimpanzees from a small-town zoo were shot and killed after they escaped from their enclosure and could not be captured, the zoo director said.

The primates at Zoo Nebraska (search) were able to get out of the cage Saturday when a padlock was not completely closed after cleaning, said zoo director Ken Schlueter Jr. (search) He killed the animals with a deputy's service revolver after a tranquilizer gun didn't show any effect.

No people were hurt, state patrol spokeswoman Deb Collins said. The zoo is located in Royal, a northeastern Nebraska (search) village of 75; one of its major donors was the late entertainer Johnny Carson.

After the chimps lifted the padlock and broke out, employees immediately moved visitors in an office area, but the chimps tried to get into the building, Schlueter said.

"When it became apparent there'd be danger here, they had to be destroyed," Schlueter told the Lincoln Journal Star.

Schlueter did not immediately return a message left for him by The Associated Press on Monday.

The Nebraska State Patrol and Antelope County Sheriff's Department were called to help capture the animals. A fourth chimpanzee also escaped, but quickly returned to its cage, officials said.

The zoo's board plans to hold an emergency meeting to discuss the escape Monday evening, board member and former zoo keeper Justin Jensen said. The chimp exhibit was a centerpiece at the small zoo.

"It's sure going to be a great loss," Jensen said.

The seven-acre zoo opened 18 years ago as the Midwest Primate Research Facility with one animal -- a chimpanzee named Reuben, one of those killed Saturday. The other three chimps all arrived at the zoo within the last two years, Jensen said.

Carson, the late host of NBC's "The Tonight Show" who grew up not far from Royal, donated $55,000 to the zoo for the Carson Center for Chimps 15 years ago and an additional $20,000 more recently. He died in January.

Schlueter said the chimps weighed up to 300 pounds or more, and he shot two of them with a tranquilizer gun, but the tranquilizers had not taken effect after five minutes.

The danger chimps pose to humans was highlighted when a man and his wife were attacked by chimpanzees March 3 at the Animal Haven Ranch near Bakersfield, Calif. The man was severely mauled and the woman lost part of her thumb before the animals where shot to death.

The Nebraska animals' carcasses were flown Sunday to a zoo in St. Louis, where autopsies were planned.