Authorities in Mexico also looking for Ariz. girl

U.S. Marshals have been in touch with Mexican law enforcement officials who are checking hotels, bus terminals and other businesses as they help look for Isabel Mercedes Celis, according to Tucson police. A photo of the first-grader is being circulated in Mexico as well.

Police began looking at the possibility that Isabel could have been taken there because of Tucson's close proximity to the border, Lt. Fabian Pacheco said.

The girl's parents reported her missing last Saturday, saying they last saw her in her bedroom the night of April 20 and discovered she was missing the next morning, according to police.

Police say a window was open with the screen pushed aside.

"We can say it's highly unlikely Isabel got up and walked out on her own. I would definitely say our investigation gears toward that — that someone is responsible for the disappearance of Isabel," Police Chief Roberto Villasenor said.

Officers also said Friday that they are searching for five possible witnesses, who appear on surveillance footage posted to police department's official Youtube channel.