But, no one suspected the ex-postal worker would go on to commit the nation's bloodiest shooting at a postal installation in nearly 20 years.
San Marco is also linked to the killing of a former neighbor.
The body of Beverly Graham, 54, was found in her condominium a day after authorities say Jennifer San Marco opened fire inside the mail sorting center where she once worked. Another woman wounded in the rampage died Wednesday, bringing the death toll to eight, including San Marco.
"The shell casings found match those found at the postal distribution center," Santa Barbara County Sheriff Jim Anderson told reporters.
He said Graham's neighbors told authorities they heard the sound of gunfire between 7:15 p.m. and 8:15 p.m. Monday night. Beginning at 9 p.m., authorities said, San Marco began shooting six postal employees and committed suicide at the Santa Barbara Processing and Distribution Center.
"She went through all the requisite screenings. There were no prior indications" of problems, said Keith Blackman, a media consultant to the Postal Service.
And in the city of Milan, New Mexico, where San Marco was a regular presence at municipal offices, village Manager Carlos Montoya told the Los Angeles Times, "We felt she was unbalanced." "Now, violent? I guess it crossed my mind. But just that she would slash a window or damage property or something," Montoya said.
Before becoming a postal worker, San Marco, 44, worked as a Santa Barbara police dispatcher in the mid-1990s and passed an extensive background check and psychological exam. She left after a few months, not unusual for a stressful job with a high turnover rate, police Lt. Paul McCaffrey told the Santa Barbara News-Press. The job did not include weapons training.
Acquaintances said San Marco, who was white, sometimes talked to herself and spewed racist comments.
Former plant worker Jeff Tabala recalled that San Marco 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.
It is unclear why San Marco killed her victims, but the U.S. Postal Inspector says that "chances are" she knew and chose her victims, CBS News affiliate KCBS reports. Santa Barbara County Sheriff Jim Anderson said it wasn't clear if the killings were racially motivated.
It is clear 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 San Marco 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."
San Marco 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 San Marco 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."
Authorities in New Mexico, where San Marco moved in 2004, also described her increasingly bizarre behavior after she lost her job. Police were contacted about San Marco 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.