An 80-year-old Seattle woman who worked as a neighborhood dog walker was killed on Tuesday morning when a man got into her car, forced her out, struck her with the vehicle and drove off with her dog, which he later killed with a knife, the police said.

The woman killed in what the authorities said was a random attack was Ruth Dalton, a great-grandmother who the police said was “well-loved.”

She was in her Subaru in the Madison Valley neighborhood with at least two dogs inside just before 10 a.m. Tuesday when a man got in from the passenger side, officials with the Seattle Police Department said at a news conference on Wednesday afternoon. The two struggled, and the moving car went off the road onto a grassy area.

Nearby drivers stopped their cars and ran to help Ms. Dalton, the police said.

One bystander backed away when the man pulled out a knife, but he returned with a stick or bat to help Ms. Dalton, the police said. By the time the bystander had run back to Ms. Dalton’s car, the man had already forced her out. The carjacker put the Subaru into reverse, crashed into two vehicles, struck Ms. Dalton and drove off, the police said.

Others who were nearby rendered first aid to Ms. Dalton, and emergency workers with the Seattle Fire Department provided medical treatment, but she was pronounced dead at the scene.