A 14-stone pet chimpanzee once seen in TV commercials mauled a woman visiting its owner and cornered a police officer in his car before being shot dead.
Captain Richard Conklin of Stamford police in Connecticut said the injured woman was taken to hospital last night in a "very serious" condition.
Capt Conklin said she suffered "a tremendous loss of blood" from serious facial injuries.
The 15-year-old chimpanzee's owner and two officers were also hurt.
Police said they had no idea why the chimpanzee, named Travis, attacked the woman as she got out of her car to visit the animal's owner, Sandra Herold.
Capt Conklin said Ms Herold wrestled with the animal then ran inside to call emergency services.
"She retrieved a large butcher knife and stabbed her long-time pet numerous times in an effort to save her friend, who was really being brutally attacked," he said.
Travis ran away and started roaming on Ms Herold's property as police arrived. Officers set up security so medics could reach the critically injured woman lying on the ground, Capt Conklin said.
As the woman was treated, the chimpanzee went after several of the officers, who retreated into their cars. Travis opened the door to one cruiser and started to get in, trapping an officer who then shot the chimpanzee several times, Capt Conklin said.
The wounded chimpanzee fled and police followed the trail of his blood down the driveway, into the open door of the home, through the house and to his living quarters, where he had retreated and died of his wounds.
Well-known around Stamford because he rode around in trucks belonging to his owners' towing company, Travis appeared on TV commercials for Old Navy and Coca-Cola when he was younger, made an appearance on the Maury Povich Show and took part in a television pilot, according to The Advocate newspaper of Stamford.
"He's been raised almost like a child by this family," Capt Conklin said. "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 behaviour at all."
Capt Conklin said the chimp had been ill from Lyme disease, "so maybe from the medications he was out of sorts. We really don't know".
Police have dealt with the animal in the past, including an incident in 2003 when he escaped from his owners' vehicle in 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, logged on to the computer to look at pictures, and watched television using the remote control, police said.