During a press conference, Lieutenant Eamon Allen said that the crime happened in the street outside the victim's residence just before noon.
Lieutenant Allen said that the victim had two children, who did not witness the crime, and sources tell KGO-TV the kids were inside the house at the time.
KGO-TV obtained law enforcement records on the incident that said, just before noon, reports came in that the woman's head was cut off by a sword. At 12:14 p.m., "Children are there, scared to come... want their mom."
CPS took custody of the 7-year-old and 1-year-old girls, along with a dog and two cats who were also inside the house.
We're told the two were in an ongoing relationship but law enforcement sources say the victim got a temporary restraining order against the suspect, Hayward resident Jose Solano Landaeta, in April.
VIDEO: San Mateo County sheriff officials discuss violent San Carlos murder
Law enforcement press conference on brutal San Carlos beheading
"They arrived within minutes and found an obviously deceased female in the street in that area," said Lt. Allen during a press conference. "They began to work the scene and shortly then after, the male suspect arrived back at the scene and was quickly detained by sheriff's deputies. He was later placed under arrest for homicide. We can confirm that a stabbing instrument was used in the commission of the crime. That weapon is still outstanding."
The search for that weapon is still ongoing. We're told the suspect suffered a medical emergency of some sort but details are limited.
Resident Terry Adams spoke with ABC7 News, hours after the murder to ask, "Why? Why did it happen?"
She and others living near the crime scene are trying to understand what could've led to Thursday's gruesome killing of their neighbor.
Broadcastify radio traffic accessed by KGO-TV alerted, "This is a 25-year-old female... head was cut off by a sword."
The Sheriff's Office only described the weapon used as a stabbing instrument, one that had not been found as of Thursday evening.
Neighbor Chapel Thorborne described the disturbing scene he encountered as he was returning home around noon Thursday, only steps from his front door.
"The head was underneath the car and she was laying in the back of the car- just severed," he described. "And they covered her up."
Thorborne said he's familiar with the couple, having briefly interacted with them a number of times. He said the last time he saw the man was just before the suspect was taken into custody.
"He was hurt, shocked, disappointed," he said about the suspect. "You can tell when he walked by, I'm like, 'Yeah, they cut somebody's head off- just laying right there.'"
Residents said their concern now is for the kids with many questions about how someone could commit such a heinous crime.
"After he cut her head off, he came walking up, him and his two friends," Thorborne said. "And they walked right by me and they arrested him."
Sources say there was a history of domestic violence at the address where the woman was killed.
Officials are providing peer support to both first responders and a group of witnesses who saw this tragic situation unfold.
There's still an active crime scene, with this community in shock. Many are only returning from work now, learning about that young mother who was beheaded in the middle of the street, in the middle of the day.
The victim's name is being withheld pending family notification.
"Anytime someone loses their life it's certainly a tragedy. As far as the shocking nature of it, I do know that the deputies that first arrived on scene were a little beset by the scene. We are providing peer support, we are also providing support for the witnesses that were on scene as well, because there were several civilian witnesses."
Love shouldn't hurt, but domestic violence can happen in any relationship - regardless of age, race, sexual orientation, religion or gender. These local resources can help.