OXFORD, Ind. -- State police detectives are taking over the search for a 12-year-old boy who has not been seen since his grandmother said she dropped him off for a camping trip in early August.

The Benton County Sheriff's Department asked that the state police take over the investigation into Nicolas Zavala's disappearance.

His grandmother reported him missing on Aug. 13 after he did not return from a camping trip for which he had left a week earlier.

Sheriff's investigators at first suspected Nicolas (pictured, left) was abducted by his mother, who lives in Virginia, but she has since passed a polygraph test.

The boy's grandmother told police she dropped him off Aug. 4 at the library in the town about 20 miles west of Lafayette for a camping trip with two friends and their uncle. Investigators have not been able to identify those friends, having checked with local schools.

The grandmother, Margaret Williams, said she could recognize one of the boys if she saw him, but is afraid she may have the wrong name.

The sheriff's department said earlier this month that they feared Nicolas may have been the victim of foul play.

Sheriff Ernie Winchester said Nicolas could have run away, but he doubts the boy would have been able to live on his own for this long.

State police Sgt. Shana Kennedy said detectives would use evidence gathered by the sheriff's department, but that they would approach it as a new case.

"Basically, we're going to start over on the investigation," Kennedy said.