VERNAL — A Vernal stepmother has been arrested for investigation of aggravated murder in the death of a toddler two months ago.

Mckenley Yadon, 24, was booked into the Uintah County Jail on Thursday, Vernal police said in a news release.

On March 14, officers were called to a home at 1536 W. 300 South on reports of an unresponsive child. The caller reported the incident as a choking, according to an affidavit filed in 8th District Court in Vernal in support of Yadon's arrest.

But the officer who responded to the home reported the 3-year-old "had been down for a period of time," police wrote. The officer saw dark purple and black bruising around the girl's left ear, the affidavit states, and "it was immediately apparent that these injuries were not consistent with the details initially provided for the medical response."

The toddler was pronounced dead shortly later at a hospital, the affidavit states.

An autopsy found the girl died from asphyxia caused by smothering, police wrote.

Court documents identify the girl by her initials, A.S. An obituary indicates her name is Arianna Stout.

Yadon and the girl's infant brother were the only other people in the home at the time of the incident, according to the affidavit.

When police arrested Yadon on Thursday, they reported that during an interview she "admitted to smothering A.S. using a blanket on A.S.'s bed," police wrote, adding that Yadon demonstrated her actions to them using a child-size CPR mannequin.

"Mckenley stated she had been planning and thinking about doing this act for two or three days prior to March 14," the affidavit states.

Yadon allegedly told police she smothered the girl for 4-5 minutes, according to the affidavit, and only stopped when the child stopped struggling.

"Mckenley advised she waited 10 to 15 minutes after A.S.'s death to contact a friend who she provided an inaccurate account of events, thus resulting in the initial 911 call," police wrote.

In addition to aggravated murder, a first-degree felony, Yadon is being investigated for child abuse, a second-degree felony.

Contributing: Annie Knox

Correction: An earlier version misidentified McKenley Yadon's gender. She is a woman.