The sanctuary said in an earlier statement that it appeared Renee Radziwon had broken its protocol by being alone in the enclosure.
The Clackamas County sheriff's office said it finished looking into the attack and concluded there was no crime to be investigated, spokesman Sgt. Robert Wurpes said.
According to the agency's report, WildCat Haven owner Michael Tuller discovered Renee Radziwon bloodied and lying on her back inside the enclosure at about 7 p.m. Saturday. Tuller pulled her by her boots into a secure entrance before calling 911.
Tuller told authorities that Radziwon was alone at the facility because he and his wife the sanctuary's executive director, Cheryl Tuller were at another property in Scotts Mills, where they plan to eventually move the sanctuary, the report said.
Responding officers found one cougar walking freely inside a main enclosure with a small amount of blood above its nose, and a second cougar in a 15-by-15 cage.
Autopsy results showed Renee Radziwon died at the scene of multiple bite wounds concentrated on her head and neck.
The sheriff's office said two cougars were inside the enclosure, but investigators were working on the theory that just one attacked the woman.
"Unfortunately it seems like a pretty cut-and-dried event," Wurpes said. "She was in the enclosure, and the cat attacked her."
The sanctuary said in a statement Sunday that while its protocol calls for two qualified workers inside an enclosure containing animals, Renee Radziwon was alone with two cats. The facility's handbook specifies that a staff member can enter an enclosure to clean or make repairs only after the animals are locked out of it.
Carol Radziwon said her daughter was very careful around the animals and would not break any safety rules, because she had written some of them herself when she started working at WildCat Haven.
Melanie Mesaros, spokeswoman for Oregon OSHA, said the agency will be investigating the incident, including looking at training, safety protocols, evidence at the scene and interviews with workers and potential witnesses.
WildCat Haven is a nonprofit that rescues wild animals such as cougars, bobcats, tigers, and other wild cats. Cougars are the size of large dogs and are native to the American West.
The sanctuary is 17 miles south of Portland, in a secluded, wooded area. The Tullers opened it in 2001 and still run it. Its website says the facility houses more than 60 cats. Renee Radziwon was the only staff member listed on the site.
WildCat Haven has an exhibitor's license, the USDA said. It is closed to the public but can conduct tours for donors.
The USDA is looking into the incident to determine whether any noncompliance with the Animal Welfare Act contributed to the attack, spokeswoman Tanya Espinosa said.
Two routine inspections conducted by the federal agency at the sanctuary in 2011 and 2012 showed no violations, according to USDA records.
Renee Radziwon, who originally was from Philadelphia, leaves behind a husband and 5-month old daughter. Her husband, Aaron Chapman, set up an online donation page for the infant.
"Renee was a devoted wife, mother and advocate for animals," Aaron Chapman wrote on the page. Her "life was taken so suddenly and tragically while doing the very thing that she cared so much about, tending to and caring for wild cats."