Parents and the public often ask why all missing children's cases do not rise to the level of an Amber Alert, like the one issued Sunday.

An Amber Alert issued in New Mexico was extended to Arizona Sunday, when a woman took off with her three sons. The four were found safe in Lancaster, California, late Sunday night.

Authorities believed the children were in imminent danger at the time.

On Saturday, the Phoenix Police Department said a 12-year-old girl and her yellow Labrador Retriever were found safe in the West Valley.

Valerie Cockrill was reported missing from her house near Bethany Home Road and 23rd Avenue Friday evening when she went to walk her dog.

Family and friends spent Saturday afternoon searching the area Valerie was last seen and posting flyers in the neighborhood. A massive social media campaign also spread on Facebook.

Police said a witness who saw her information on the news found her in the area of 26th and Georgia avenues.

Phoenix Police reached out to 12 News and other media outlets to broadcast her picture and information. Despite the teen being gone for more than 24 hours an Amber Alert was never issued.

The reason is simple and yet sometimes frustrating to worried parents. The system works in coordination with the U.S. Department of Justice. In Arizona DPS issues an alert and specific criteria are in place:

"The Arizona AMBER Alert Plan requires law enforcement to meet the following criteria when evaluating a child abduction. Law Enforcement Agencies must have all the items listed below before activation can occur. The guidelines are as follows:

1. An abduction of a child (under 18) has occurred.
2. The abduction process poses a credible threat of immediate danger of serious bodily injury of death to the child.
3. A law enforcement agency has determined that the child is not a runaway and has not been abducted as a result of a child custody dispute, unless the dispute poses a credible and or specific threat of serious bodily harm or death to the child."

Surprise Police are looking for a teenager girl that ran away from home, the department said Monday. The department sent out a tweet with the teen's picture and is asking everyone to be on the lookout. However, since she is considered a runaway no Amber Alert has been issued.

In 2015 Arizona issued four Amber Alerts, that same year there were 182 nationwide.