PHOENIX – A mother accused of smothering her three children in Arizona has been formally indicted on three counts of first-degree murder.

The charges were filed against 22-year-old Rachel Henry of Phoenix on Tuesday after a grand jury reviewed the case. Henry was arrested last week after police said she admitted to killing her three children under the age of 4 on Jan. 20.

"This office is committed to seeking justice on behalf of the three young lives lost in this horrific tragedy," Maricopa County Attorney Allister Adel said in a statement. "I want to thank the first responders and prosecutors who responded on January 20, as well as those who have carried this case forward and will continue to do so.”

Court records say Henry told police that she sang to her children while she killed them and then placed their bodies in the living room as though they were sleeping. The children were identified as 3-year-old Zane Ezri Henry, 23-month-old Mireya Henry and 6-month-old Catalaya Kyeana Rios.

Henry will appear in court for an arraignment on Feb. 4. She's being held in a Maricopa County jail on $3 million bail.

Follow Bree Burkitt on Twitter: @breeburkitt.