A former plant worker said the attacker, 44-year-old Jennifer Sanmarco, had made racist comments in the past, and at least six of the victims were minorities, but investigators have refused to discuss a motive in the slayings.
The eighth victim, Beverly Graham, 54, was found Tuesday, dead of a gunshot wound to the head, at a Santa Barbara condominium complex where the former postal employee lived up until a few years ago.
Sheriff's Sgt. Erik Raney said authorities believed Graham's death was "the beginning of this rampage."
It is unclear why Sanmarco killed her victims, but the U.S. Postal Inspector says that "chances are" she knew and chose her victims, CBS News affiliate KCBS reports.
A neighbor of Graham's reported hearing a gunshot Monday evening, before Sanmarco went to the mail-processing center.
Sanmarco shot six postal employees to death and committed suicide in what was believed to be the nation's deadliest workplace shooting by a woman. It was also the nation's bloodiest shooting at a postal installation in nearly 20 years.
A postal worker who was shot in the head, Charlotte Colton, 44, died of her wounds Wednesday, said Teresa Rounds, spokeswoman for Santa Barbara Cottage Hospital.
Former plant worker Jeff Tabala recalled that Sanmarco seemed particularly hostile to Asians while working for the Postal Service. He said all of the dead were minorities: Three were black, one was Chinese-American, one was Hispanic and one was Filipino.
The shooter had a long history of mental problems, officials said. She had been placed on a medical leave from her postal job for psychological problems.
In 2003, Tabala said, he saw sheriff's deputies pull Sanmarco out from under a mail-sorting machine and wheel her away in handcuffs on a mail cart after a disturbance.
She returned several months later but "people started coming to me and saying, 'She's acting erratically,"' Tabala said. "She was screaming. She was saying a lot of racist comments. It was pretty ugly."
Sanmarco was escorted out of the building by management and never returned, Tabala said.
"She seemed to be having conversations and there wasn't anyone around her. She'd be just jabbering away." Tabala said.
Graham's boyfriend, Eddie Blomfield, said Sanmarco would often go outside singing loudly, which led to arguments between the women. Graham's brother Les Graham Jr. said his sister had complained about a woman who "used to come out and rant and rave in front of her building."
Investigators gave no motive for the postal rampage but said Sanmarco had been put on medical leave in 2003 for psychological reasons and had to be removed from the building once.
Authorities in New Mexico, where Sanmarco moved in 2004, also described her increasingly bizarre behavior after she lost her job. Police were contacted about Sanmarco at least twice after she was accused of harassing an office worker and appearing naked at a gas station. She was dressed when officers arrived.
A deputy clerk for the city of Milan, N.M., said Sanmarco applied for a business license in 2004 for a publication called "The Racist Press" that she said she planned to launch.
"We weren't sure what she was going to do next," said Terri Gallegos, deputy clerk for the city of Milan, New Mexico, where Sanmarco applied for a business license in 2004 for a publication called "The Racist Press" that she said she planned to launch. Another time she said she wanted to register a cat food business.