Joggers found the body of April Millsap, 14, July 24 in a drainage ditch in Armada, Mich., 35 miles northeast of Detroit. The ditch was near the Macomb Orchard Trail, a hiking and biking trail on an old rail line.
The joggers were led to the body by April's dog, which was roaming the area. She had taken her pet for a walk along the trail a few hours earlier.
Police said the text message appeared to be the last sign April was alive.
Investigators have not said how April was killed, although they said she was not shot or stabbed.