A Fort Bragg woman who suffered from dementia died Monday night when she was struck from behind by a pickup while walking down Highway 1, according to Ukiah CHP and a family member.

Joanna Cohen, 79, was walking south in the middle of the southbound lane of Highway 1 south of Ocean Drive around 8 p.m. Monday, the CHP reported.

The area was dark, with no streetlights, the CHP said.

Cohen did not see the pickup as it approached from behind her, CHP officials said. The driver of the pickup, 18-year-old Troy Arter III of Fort Bragg, was traveling about 45 mph and did not see the woman until he was too close to avoid a collision.

After hitting her, Arter immediately pulled to the right shoulder of the road. Cohen was pronounced dead at the scene by emergency responders, the CHP said.

The cause of the crash is under investigation, although alcohol was not a factor for the driver, the CHP said.

Cohen began showing signs of dementia about five years ago and her symptoms got worse beginning about a year ago, said Linn Turner, her partner of 35 years.

The couple lived in Mendocino for years and after losing their home to foreclosure moved to an age-restricted mobile home park outside Fort Bragg, said Turner, who operates a locksmith business.

In recent weeks, the dementia made Cohen unable to take care of herself and distrustful of her long-time partner, Turner said. Social services got involved and Turner was forced on Thursday to leave the home the couple shared.

The Mendocino County Sheriff's Office was not able to comment immediately on the case Tuesday night.

Turner said she was staying in a Fort Bragg hotel when she learned Tuesday morning that her partner had been killed. She said she had warned officials from Adult Protective Services that Cohen couldn't be left alone.

"I really thought someone was going to be there, and they weren't," she said.

When she was healthy, Cohen was an extremely intelligent woman with a quick wit and love of animals, Turner said. For a period she ran a computer repair business in Mendocino. Cohen refused to see a doctor, and when her health deteriorated, so did their lives, finances and relationship, Turner said.

"This is a terrible disease. Absolutely terrible," Turner said.