The girl is believed to be from 4 to 6 years old, Saginaw Police Chief Roger Macon said. Her identity has not been confirmed, although police received a separate call about a missing girl from a family less than two miles away from where the body was found Monday evening, Macon said, according to the Fort Worth Star-Telegram.
The Tarrant County medical examiner's office was expected to perform an autopsy on Tuesday.
The FBI and Texas Rangers are involved in the investigation.
One of the two teens who said they found the body told Dallas television station WFAA that the girl's hands and feet were tied and a plastic shopping bag was placed over her head.
"I dropped to my knees crying because I knew it was a little girl," said the teen, John Smith.
Saginaw Officer Damon Ing said some area residents reported seeing a red truck in the area with a green paint splatter near the right back tire.
Saginaw, population about 17,000, is just north of Fort Worth.
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