Sandra Herold, who owned the 15-year-old chimp named Travis, wrestled with the animal, stabbed it and hit it with a shovel after it inexplicably attacked her friend Charla Nash, 55.

Nash had gone to Herold’s home in Stamford on Monday to help her coax the chimp back into the house after he got out, police said. After the animal lunged at Nash when she got out of her car, Herold ran inside to call 911 and returned armed.

“She retrieved a large butcher knife and stabbed her longtime pet numerous times in an effort to save her friend, who was really being brutally attacked,” said Stamford police Capt. Richard Conklin. Herold told police that the knife had no effect, and that she also struck Travis with a shovel.

Nash was in critical condition Tuesday after suffering what Stamford Mayor Dannel Malloy called “life-changing, if not life-threatening,” injuries to her face and hands.

Her sister-in-law, Kate Nash, said Tuesday morning that Nash underwent surgery Monday night and came out of it “OK.”

Herold and two officers also received minor injuries, police said. Conklin said police don’t know what triggered the attack.

“There was no provocation that we know of. One thing that we’re looking into is that we understand the chimpanzee has Lyme disease and has been ill from that, so maybe from the medications he was out of sorts. We really don’t know,” Conklin said.

Colleen McCann, a primatologist at the Bronx Zoo, said Tuesday that chimpanzees are unpredictable and dangerous even after living among humans for years.

“It’s deceiving to think that if any animal is … well-behaved around humans, that means there is no risk involved to humans for potential outbursts of behavior,” she said. “They are unpredictable, and in instances like this you cannot control that behavior or prevent it from happening if it is in a private home.”

After the initial attack, Travis ran away and started roaming Herold’s property until police arrived, setting up security so medics could reach the critically injured woman, Conklin said.

But the chimpanzee returned and went after several of the officers, who retreated into their cars, Conklin said. An officer shot Travis several times after the animal opened the door to his cruiser and started to get in.

“The animal had cornered him,” Conklin said Tuesday. “He had no other recourse.”  The wounded chimpanzee fled into the house and retreated to his living quarters, where he died.

A woman answering the door at Herold’s house Tuesday morning declined to comment.

Conklin told reporters the chimp was acting so agitated earlier that afternoon that Herold gave him the anti-anxiety drug Xanax in some tea. Conklin also suggested the animal may have attacked Nash because she was wearing her hair differently and perhaps wasn’t recognized.

The chimpanzee was well-known around Stamford because he rode around in trucks belonging to the towing company operated by his owners.

Police have dealt with him in the past, including an incident in 2003 when he escaped from his owners’ vehicle in downtown Stamford for two hours. Officers used cookies, macadamia treats and ice cream in an attempt to lure him, but subdued him only after he became too tired to resist.

At the time of the 2003 incident, police said the Herolds told them the chimpanzee was toilet trained, dressed himself, took his own bath, ate at the table and drank wine from a stemmed glass. He also brushed his teeth using a Water Pik, logged onto the computer to look at pictures, and watched television using the remote control, police said.

When he was younger, Travis appeared on TV commercials for Old Navy and Coca-Cola, made an appearance on the “Maury Povich Show” and took part in a television pilot, according to a 2003 story in The Advocate newspaper of Stamford.

“He’s been raised almost like a child by this family,” Conklin said Monday. “He rides in a car every day, he opens doors, he’s a very unique animal in that aspect. We have no indication of what provoked this behavior at all.”