"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 in February when a 140-pound jaguar named Jorge got into an employee access hallway through the open cage door and pounced on her. An autopsy found she died of a broken neck and had extensive internal injuries.
Piper said Pfaff had broken at least two zoo rules by failing to keep two locked doors between herself and the jaguar, and by failing to verify the location of the jaguar before opening an access door.
The jaguar was shot and wounded and later euthanized by other zoo employees.
Police concluded two weeks ago that the attack was "a tragic accident" but 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 jaguar's cage door nearly wide open and its padlock lying on the floor beneath it.
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.
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 immediatley 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.
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.
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