Border Patrol agents got the information on her movements from two women from India who told officials they'd been separated from a woman and two children traveling in their group hours earlier.
"Agents took the two women into custody and began searching the area north of the international border in remote terrain seven miles west of Quitobaquito Springs," the agency said in a statement. "Within hours, they discovered the little girl's remains."
Agents used helicopters to search for the people she'd been traveling with, and found footprints indicating they returned to Mexico.
"Our sympathies are with this little girl and her family," Tucson Chief Patrol Agent Roy Villareal said. "This is a senseless death driven by cartels who are profiting from putting lives at risk."
Customs and Border Protection agents along with Mexican authorities are searching both sides of the border for anyone associated with the group.
A decade ago, in 2009, that number was 204.