The 32-year-old had been in and out of hospital and was on medication for depression and possible bipolar disorder. So, in late 2012, her mother started spending nights at the family's Edmonton apartment to help look after the kids. Sarmiento's husband thought they would be safe.
But on Feb. 12, after her husband and mother left for work, Sarmiento sent her 10-year-old daughter off to school and started filling the bathtub with water.
Seven-year-old Omar Jajoy had been sitting at the kitchen table eating breakfast and finishing his homework.
Court documents show his mother shoved him into the bathroom, pushed him into the tub, got into the water herself and held his small head down while he thrashed in the water.
After he stopped moving, she called 911.
A court hearing began Thursday to determine if Sarmiento should be held not criminally responsible for the crime.
The woman admits in court documents that she killed her son but she has pleaded not guilty to a charge of first-degree murder. Her defence lawyer, Peter Royal, plans to call medical evidence showing she was suffering from a mental illness at the time.
In a videotaped interview with police after the drowning, Sarmiento said she knew that killing her son was wrong and she must pay for what she had done.
She said she had wanted to drown both her children that day. But her daughter was too big.
Sarmiento described how the family moved to Canada from Colombia in 2007. She found it lonely and difficult to learn English. For two years before the drowning, she saw a doctor and was taking monthly injections so she could feel better. Her medication changed several times.
"I have been sick, my mind plays with me," she told police. "Sometimes I imagine things."
Sarmiento spoke about how she was worried about the family's finances and the kids always needed new clothes, shoes and socks. The morning of the killing, she heard her daughter say they were "hobos."
"Is it worse to be dead or is it worse to be poor?" asked a police officer.
"I think it's worse to be alive," Sarmiento replied.
She said she tried to hang herself the day before she drowned her son. She had planned to try again after he died in the tub but said she mistakenly dialled 911 too soon.
Sarmiento's husband, Florentino Jajoy, wasn't in court for the hearing. But he earlier told reporters his wife wasn't to blame for his son's death. The medical care she had been getting was obviously "wrong," he said, and the health system failed her.
It's not clear what Sarmiento disclosed to her doctor.
She told police she had thoughts of stabbing the children and smothering them with a pillow. She once fantasized about throwing them over a third-floor railing at a downtown shopping centre.
Court documents show that after she told her mother, Maria Cristano, about the deadly thoughts bouncing around her head, the older woman questioned her. Sarmiento told her that her doctor knew about it.
When she told her husband, he threatened to call police and a crisis team. But, after she got some sleep, she reassured him that everything was OK and she wouldn't harm the children.
But at some point in 2012, Sarmiento took her daughter into a bedroom and locked the door. She put the girl on a bed and started choking her around the neck.
"She said it was a game," the girl told police in another videotaped interview.
"I'm like, 'That's not a game Mommy.' And I started crying ... I'm like, 'Mommy, please get off me!' And she got off me and she said, 'I'm sorry.'"
The girl said her mother asked her not to tell anyone or she would have to go back to the hospital.
After the police detective left the interview room, the video shows the girl quietly talking to her dead brother.
"Omar, if you're up in the sky or here on Earth, can you take care of yourself," she said.
"Please pray, brush your teeth and (do) everything that you need. I love you very much."
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