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 were able to get out of the cage Saturday when a padlock was not completely closed after cleaning, said zoo director Ken Schlueter Jr. 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 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 into 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.