"This was a horrific death," said U.S. Attorney Jenny Durkan. "These poor children suffered tremendously."
Durkan said Christina Carlson's kids were strapped into car seats and practically abandoned for days on the Tulalip Indian reservation by their own mother.
"There was a long period of neglect for these kids," Durkan said. "There's obviously substance abuse issues. There was a lot going on."
The breaking point came in October when police found 19-month-old Chantel Craig dying near the car with her 3-year-old sister still inside.
An autopsy showed Carlson's daughter Chantel suffered from dehydration and severe malnutrition, weighing just 18 pounds with prominent ribs and a sunken abdomen. She was found covered in urine and feces, with maggots in her diaper and lice in her hair.
Her older sister was found in a similar condition - pale but still alive - strapped into a car seat with abrasions on her skin and bleeding wounds. Her blanket was found to be infested with maggot larva and bed bugs.
Court papers say the mom had left them to score drugs.
"To die like that, I think is just tragic," said Chris Case with Department of Health and Human Services.
Child welfare workers say they'd been tracking Carlson for years because of neglect complaints, but she actively avoided them. Case says when they finally did catch up with Carlson, she'd cleaned up the house and kids -- at least for that visit.
"What happened after that was we couldn't find her again," Case said.
Social workers say they kept searching for months without luck, stymied in part by a lack of access on the reservation.
In a tragic twist, the day DSHS closed the case, Carlson resurfaced -- this time with a dead baby. A child fatality review found DSHS acted appropriately despite the outcome.
" 'Was this reasonable? Did you do everything?' and the committee agreed with that that we had done everything reasonable," Case said.
Durkan says they want to make sure everyone is on the same page.
"We really want to make sure were working with the tribes, with the state, to the best we can, to make sure these cases don't happen again," she said.
Carlson remains in custody at the federal detention center with her murder trial set to begin on July 22. If convicted, Carlson could face a maximum sentence of life in prison.