It happened Saturday when the woman crashed an SUV into a bus shelter on Ulloa Street. Jail records show she had no prior criminal or civil cases in San Francisco.
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"I was doing my laundry and heard a loud crash, I looked out the window and saw this mangled car," said neighbor Joanna Dimayuga.
Dimayuga raced to help.
"I saw a lady being attended to, I gave her a towel. She had a bloody mouth, I later found out that was the driver," Dimayuga added.
Investigators say the woman was speeding, driving the wrong way on Ulloa street, traveling east In the westbound lane, before slamming into the bus stop in front of the West Portal Library.
VIDEO: 78 year-old SF woman arrested for crash which killed 3 family members
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Simmone Fichtner is the woman's neighbor and says she can't believe the news.
"She's always a calm woman, she says hello and she watches her grandkids. It's very unexpected, I'm so sorry," Fichtner added.
Friends of the couple killed say they were full of life. They said on Saturday, the family was headed to the zoo and decided to take the bus.
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VIDEO: Neighbors gather around memorial for 3 family members killed after SF bus crash
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Family friends tell ABC7 News four people, a couple with their toddler and baby, were hit.
The couple and the toddler died, and the baby has life-threatening injuries.
The community has been rocked by this tragedy.
With debris from the crash picked up, items left in memory of the family have been placed at the scene.
Loved ones and neighbors are dropping off flowers along with candles and stuffed animals.
Kristi Bulnes lives in the neighborhood and dropped off flowers at the memorial. She says she plans on going to the vigil.
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"Obviously this story touches my heart," she said. "I have a 2-year-old. I have an older child and we're in this neighborhood all day every day, multiple times a day, and it could've been anybody so I wanted to share my condolences."
On Monday night, hundreds attended a vigil for the family in San Francisco. The San Francisco mayor and police chief were also in attendance.
Pictures showing the family were put up by close friends in the area where the accident happened.
"This is the most graphic, vivid, and gruesome thing I've ever seen in my life," said a woman who witnessed the accident.
Some neighbors say that whatever the cause of the accident may be, the area in the West Portal neighborhood where this happened is very confusing, and they believe changes need to be made.
"People don't know where to walk, not to walk, you can see the lights are terrible. People don't know when to cross the street!" said Joe Kaplan who lives nearby.
Jodie Medeiros from Walk San Francisco, a nonprofit advocating pedestrian safety, is heartbroken by the crash. Her group is planning the vigil at the crash site.
"Nobody should die this way. Our community shouldn't be losing members of our community this way. It's absolutely devastating," Medeiros said.
RELATED: 1 child, 1 adult killed when SUV plows into SF West Portal bus shelter, SFFD says
Medeiros points to Vision Zero, San Francisco's goal to eliminate traffic-related deaths and injuries.
"The fact we're not there means we need to be doing more, and the city needs to do better," Medeiros said.
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San Francisco resident Luke Bornheimer started a petition after hearing about the crash.
"My petition is calling for the closure of the intersection of West Portal Ave and Ulloa St to car traffic," Bornheimer said. "That's to make it safer for everyone who uses this area and make public transportation more reliable and safe."
Organizers of the vigil say it's about holding the community close.
District 7 Supervisor Myrna Melgar says they're working on up putting signage in the area.
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San Francisco District Attorney Brooke Jenkins released a statement on Monday writing:
"I offer my heart felt and deepest condolences to the family and friends of the victims and the West Portal community that has been so deeply impacted by the crash on Saturday afternoon. There are no words to adequately describe this tragedy. My office's Victim Services Division has been in touch with the victims' family and will work with them closely. The family has asked for privacy as they grieve and work to make sense of what has occurred.
Although the San Francisco Police made an arrest in this case, this remains an active and ongoing investigation that is only in its very beginning stages. The police had probable cause to arrest (suspect) based on the preliminary evidence gathered to date. My office is now in the process of receiving case information and evidence from the police department.
While I understand that there is an immediate desire to know what ultimately will happen in this case, I ask for patience from the public as investigators continue to gather more information so that my office can make appropriate charging decisions. The extent and nature of any criminal liability depends on many factors. For example, toxicology results alone can take more than 30 days to obtain and, vehicle operating systems will need to be carefully analyzed by experts and assessed to determine whether mechanical issues may have played a role in this incident. These are just two examples of the continued investigation that must occur and cannot be completed within the 48 hours that we legally have to render a charging decision.
My office will continue to stand with and support the victims' family and the community during this difficult time."
INTERACTIVE: Take a look at the ABC7 Neighborhood Safety Tracker