Tulalip police on Wednesday continued to investigate the death of 17-month-old Chantal Craig, who was found dead in the back of a car in Marysville on Monday.
The girl's older sister was found alive but unconscious and was reported in stable condition at Children's Hospital in Seattle. The mother is being held on investigation of child neglect and endangerment.
People living near where two girls were found said the area is a known gathering place for drug users. They also said that the mother was recently asked to leave a home where she was staying because of the company she was keeping.
Child Protective Services confirmed it had conducted a joint investigation of Chantal Craig's mother with Tulalip Police, but the agency did not remove the two girls from the mother's custody.
The agency did not reveal when that investigation happened, saying only that it was "recently." The children were not removed because investigators couldn't obtain sufficient information about the girls' situation.
Tulalip Police Chief Rance Sutten said CPS and his agency "would've taken action" had their investigation uncovered grounds for removing the girls.
Sutten said police have interviewed the girls' father and do not consider him a suspect in the case.
The mother is scheduled to make her first appearance in court on Thursday.