TUCSON, Ariz. â€"  The search for a 6-year-old Arizona girl who disappeared from her family's home has extended to Mexico, authorities said Friday.

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.

The video, which runs a little more than a minute, shows two males and three females -- walking in a parking lot in the direction of the Celis home last Saturday between 1 and 1:30 a.m.

Villasenor said they are hoping to identify anyone on the video in case they are potential witnesses or have information useful to the investigation.

Villasenor advised the public to come forward if they notice anyone who showed changes in normal activities or behavior Saturday or the days since.

Investigators spent Thursday night combing through the lake at a local park with sonar equipment. They said they were acting on a tip, but wouldn't elaborate.

Officers also searched bodies of water in other city parks.

Pacheco said police are also looking for any video from a baseball game that Isabel's family attended the night before she disappeared.