An Arizona woman was arrested for murder on Sunday on suspicion of drowning her 2-year-old twin sons in a bathtub and attempting to do the same with another child, authorities said.

Mireya Lopez, 22, was taken into custody at the suburban residence west of Phoenix facing two counts of first-degree murder and one count of attempted murder, police said.

“She was very open and honest with our investigators that she did it,” said Sgt. Brandon Busse, an Avondale, Arizona, police spokesman. The motive, however, was not immediately clear.

The boys’ grandmother placed a 911 call at about noon saying that the children were not breathing. Busse said responding officers found the twin boys unconscious on a bed and began resuscitation efforts before they were taken to two local hospitals, where they were pronounced dead.

Lopez, who was being held in a Phoenix jail, also told investigators that she tried to drown another child but was thwarted by a family member, Busse said. The child was uninjured. It was unknown whether Lopez was the child’s mother.

Autopsies will be performed to determine the official cause of the deaths.