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PHOENIX, Ariz. (NBC) – Phoenix police said the mother of the three children found dead in their home on Monday has admitted to killing them and has been arrested.

Police said it was a 3-year-old boy, a 2-year-old girl and a 7-month-old girl that died near 24th Street and Southern Avenue Monday evening.

The mother was identified Tuesday as Rachel Henry, 22. She was been booked on three counts of first-degree murder.

Police have not yet said how the children died.

“This investigation is still ongoing, there’s still a lot of collection and gathering of evidence that still needs to be done,” Fortune said. “This case is very complex, there’s a lot of moving parts to it.”

The children were found in the living room of the home near the entrance.

“We didn’t have to go looking for the children,” Fortune told reporters Tuesday.

Police say the mother is a 22-year-old woman who recently moved to Arizona from Oklahoma.

Firefighters responded to what was initially a drowning call and attempted lifesaving efforts, but the children all died at the scene.

The parents and another adult relative were all in the home at the time, police said.

Police said initially there were no obvious signs of trauma, but investigators were interviewing the mom and dad to find out how the children died.

A family member told officers the children were sick earlier Monday night, according to police.

The fire crews that responded to the scene were sent home for the rest of the night because of the traumatic nature of the situation.