"Evidence and circumstances of both crimes show distinct correlations between the two," said Jeff Klapakis of the Santa Barbara County sheriff's department.
Meanwhile, a hospital official says a sixth postal worker shot by the former employee had been in critical condition, but has died.
The shooter, Jennifer Sanmarco, 44, had a long history of mental problems and bizarre behavior, officials said. She had been placed on a medical leave from her postal job for psychological problems.
The Santa Barbara News-Press reported that police were looking into the separate slaying of a 54-year-old woman who was found shot to death in her Goleta apartment. The victim was a former neighbor of Sanmarco who had known the shooter.
The Press-News said police found a 9mm shell casing next to the former neighbor's body. Sanmarco used a 9mm handgun at the mail-processing facility where she once worked.
The newspaper quoted the ex-neighbor's boyfriend, Edward Blomfield, as saying that Sanmarco was "insane."
Blomfield said the dead woman had a mostly friendly relationship with Sanmarco, but that the pair had sometimes quarreled because of Sanmarco's bizarre behavior, which included loud, late-night singing.
The former neighbor was identified by her brother as Beverly Graham, 54.
Les Graham said his sister had complained about a neighbor who "used to come out and rant and rave in front of her building." He said the family suspects that the neighbor and his sister's killer was Sanmarco.
Investigators would not discuss a motive for Monday night's postal attack. It was believed to be the nation's deadliest workplace shooting ever carried out by a woman.
Through interviews with authorities and people who knew Sanmarco, a picture emerged of a woman on an odyssey of increasingly bizarre behavior after losing her job.
"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.
During one meeting, Gallegos said, Sanmarco carried on a conversation with herself "like she was arguing with someone but there was no one there."
Last March, office workers called authorities after the 44-year-old woman made what Gallegos described as a rude allegation. Other times, Gallegos said, Sanmarco would come in and simply stare at one employee in particular.
In June, police talked to her after someone at a gas station called to complain of nudity, Police Chief Marty Vigil said. Sanmarco was dressed when officers arrived.
U.S. Postal Inspector Randy DeGasperin told reporters Tuesday that Sanmarco left the mail facility on a medical leave in 2003 after her co-workers expressed concerns she might hurt herself. He said police removed her from the building once.
"She was not making any threats or anything of that nature," DeGasperin said. "It was more for her safety."
Authorities said it was unclear whether Sanmarco targeted specific employees when she arrived at the postal center about 9 p.m. Monday. But DeGasperin said "chances are" she knew the people she was shooting at.
"According to witnesses from the scene, she had a 9mm pistol and reloaded at least once during her rampage," said Santa Barbara County Sheriff James Anderson.
Killed were Ze Fairchild, 37, and Maleka Higgins, 28, both of Santa Barbara; Nicola Grant, 42, and Guadalupe Swartz, 52, both of Lompoc; and Dexter Shannon, 57, of Oxnard. Charlotte Colton, 44, of Santa Barbara, died Wednesday after being hospitalized in critical condition.
Higgins had just returned from maternity leave about a month ago and leaves behind a baby girl and her husband.
"She was a talker. There was not a moment she was quiet," said colleague and friend Lexi Bushnell told the Santa Barbara News-Press. "She loved to lighten things up."
Swartz was emerging from a dark period after losing her husband, Donald, three years ago to cancer, according to friend Darlene Skura.
"She was becoming more active, starting to get on with her life," Skura told the Los Angeles Times in Wednesday's editions.
Grant's neighbors said it was not uncommon to see the married mother of two playing basketball with her children.
"She was such a joy," said friend and neighbor Leslie Brown. "When you talked to her, she just glowed."
Police said Sanmarco entered the sprawling Santa Barbara Processing and Distribution Center by driving through a gate behind another car. She gained entry to the building by taking an employee's identification badge at gunpoint. That worker was not hurt.
Only about 80 of the approximately 300 people who work at the mail-sorting center were on hand when Sanmarco arrived. Authorities said many of them fled to a fire station across the street when the shooting began.
"I was dumping mail on a belt when the gunshots suddenly (went) 'Boom! Boom! Boom! Boom!" said postal worker Alger Busante, 56, of Santa Barbara.
It was the deadliest shooting at any workplace since 2003, when 48-year-old Doug Williams gunned down 14 co-workers, killing six, at a Lockheed Martin aircraft parts plant in Meridian, Miss., before turning the gun on himself.