The Lincoln Park Zoo has paid a $3,000 fine after federal authorities ruled it was to blame for a gorilla attack on a zookeeper and the deaths of several monkeys last year, officials said.

The matter is considered closed by the U.S. Department of Agriculture and there are no further investigations of the zoo, agency spokesman Darby Holladay said.

Scrutiny began after the deaths of three elephants, two gorillas and a camel over the past two years — incidents that led animal rights groups to protest outside the zoo.

But the USDA's Animal and Plant Health Inspection Service only held the zoo responsible for the deaths of three Francois langur monkeys in May 2005 and the gorilla attack two months later. The fine was imposed in February.

The zoo did not have proper barriers in place at the gorilla habitat and didn't have adequate enclosures for the monkeys, Holladay said.

"They followed all regulations regarding all the other animal deaths," he added.

Lincoln Park Zoo spokeswoman Kelly McGrath said the three monkeys died after eating leaves from a yew tree just outside their habitat.

"We knew a mistake had been made, and we immediately removed the plant not only from the exhibit but from the zoo grounds," she said.

An employee's error led to the zookeeper being bitten by the gorilla, she said.

People for the Ethical Treatment of Animals said other animals at the zoo could be at risk.

"The fact that the zoo was fined — a rarity, to say the least — for failing to maintain the woefully minimal standards of the Animal Welfare Act does not bode well for the animals who still live there," PETA director Debbie Leahy said in a statement Tuesday.