A California woman is suspected of killing her two daughters and then trying to take her own life in a failed suicide attempt, police said.

The children, a 14-year-old and a 4-month-old, were found dead on Tuesday by their father at their home in Ontario, a city east of Los Angeles, according to Ontario Police Department Sgt. Bill Russell.

The father returned home around 4 p.m. and found his daughters in the garage, the department said in a press release. Attempts to revive them were not successful.

The children's mother, identified by police as 47-year-old Linda Nguyen, was found at the home injured and unresponsive.

Two sisters were found dead in the garage of a home in Ontario, California. NBC Los Angeles

Russell said at a news conference on Wednesday that two suicide notes were discovered; one in a car and another inside the home.

He declined to discuss what the notes said or if they indicated a motive for the murders but told reporters that the mother had been dealing with depression.

He also said that there were signs of a struggle at the home and both children had "visible injuries." The teenage daughter had special needs, Russell said without elaborating.

Linda Nguyen, age 47. Ontario Police Department

Authorities have not said how the children died and a cause of death is still being determined.

Police said Wednesday that Nguyen was in the hospital and had been unresponsive due to her medical condition. In a statement posted on Twitter Thursday night, Ontario police said she had been placed under arrest and would be booked at the West Valley Detention Center on two counts of murder once she was medically cleared.

Russell said the girls' father is understandably "distraught."

"He’s lost both of his children and his wife is in the hospital," Russell said. "He lost everything.”