DENVER -- A Denver Zoo keeper who was attacked and killed by a jaguar had violated the rules by opening the door to the animal's cage, zoo officials said Tuesday.

Zoo Vice President Craig Piper said the February death of Ashlee Pfaff was caused by "human error."

"Our investigation shows Ashlee did not follow established safety protocols on the day of the accident," Piper said.

He called her death "a true tragedy."

Pfaff, 27, was killed on Feb. 24, when a 140-pound jaguar named Jorge got into an employee access hallway through an open cage door and pounced on her. An autopsy found Pfaff died of a broken neck and had extensive internal injuries.

The report concluded that Pfaff had broken at least two zoo rules by failing to keep two locked doors between herself and the jaguar, and by failing to visually verify the location of the jaguar before opening an access door.

"The first thing that a keeper is trained to do before doing any transfer is to visually verify the location of their animal. This is the fundamental rule of zookeeping -- always know where your animals are," Piper said.

An zoo witness and eyewitness to the fatal mauling told the zoo that Pfaff had opened the wired mesh door to an indoor animal habitat while the male jaguar was inside. The witness said that the jaguar was about 2-3 feet from that mesh door, facing the door, and had to be in Pfaff's direct line of sight.

"We know that Ashlee breached at least two well-established safety protocols. And we do not know why she did that," said Denver Zoo spokeswoman Ana Bowie.

"When the witness turned back to face the exhibit, that witness shared that the jaguar had moved. And it took a moment to realize that the jaguar was partially inside the habitat and partially outside. And it took another moment to realize that the jaguar had a person, right there," Piper said.

The jaguar was shot, wounded and later euthanized by a zoo veterinarian.

Police concluded two weeks ago that the attack was "a tragic accident," but investigators did not address why the cage door was open.

Zoo officials found no faulty doors, locks or gates. One zoo worker told police that after the attack, he found the door to the jaguar's cage nearly wide open and its padlock lying on the floor beneath it. A Denver police investigation found no criminal wrongdoing in the death.

The conclusions of the zoo's internal investigation said Pfaff may have failed to check the door or, more likely, failed to close it properly.

"We cannot determine any specific reason that caused her to fail to follow established protocols that she was adequately trained to perform and performed regularly without incident for over one year at Denver Zoo," the report said.

There are no video monitoring systems that can answer why Pfaff opened the door and when she had opened it, but zoo officials may consider installing a system in the future.

Piper said a copy of the report was sent to the woman's family in New Mexico.

Pfaff's parents, Norman and Janice Pfaff of Rio Rancho, N.M., did not immediately return a phone message left by The Associated Press seeking comment.

Piper said the animal display will remain closed to the public so her co-workers can grieve. He said the zoo not only lost a valued worker, but a "magnificent creature" as well. The exhibit has been opened for 40 years without an accident.

Pfaff's death is the zoo's first fatality involving an animal in 80 years.

Pfaff's death is also being investigated by the U.S. Agriculture Department, the Occupational Safety and Health Administration and the Association of Zoos and Aquariums, a zoo accrediting agency.

Pfaff, who had started work at the Denver Zoo in 2005, graduated from New Mexico State University in 2002 with a degree in biology.

The Denver Zoo obtained Jorge in 2005 from the Santa Cruz Municipal Zoo of South American Fauna in Bolivia.