Clayton County Police Chief Mike Register said the 15-year-old boy was located in Chattanooga, Tennessee, where he fled before gang members shot 11-year-old Tatiyana Coates and her 15-year-old brother Daveon Coates as they slept in their Jonesboro, Ga., home on Oct. 22.
Investigators located the teenager at 3:45 p.m. on Tuesday in Chattanooga, and began questioning him about the murders, the Times Free Press reported.
The boy is not a suspect in the double homicides.
Chief Register said the gang members who entered the Jonesboro home were actually looking for the 15-year-old boy who was not home.
The mother of Tatiyana and Daveon invited a single mom with 5 children -- including the teenage boy -- to move into her Jonesboro, Ga., home from Chattanooga a couple of months ago.
Police say the unidentified teen quickly became involved in violent gang activity in Atlanta.
"The 15-year-old who came with the second family got involved in gang activity here in metro Atlanta," Register said. "We believe there was an act that he committed against a local gang that made them retaliate in such a vicious manner."
The teenager fled to Chattanooga before the murders occurred, Register said.
Investigators believe local gang members pursued the boy to Chattanooga, where he may have been involved in a shootout between two vehicles in the 500 block of N. Hawthorne Avenue around 4 p.m. on Sunday, a day after the Coates siblings were killed.
"It appears that one of the local gangs that we believe was involved in the murders of the young girl and boy had traveled to Chattanooga, and the [shootout] that occurred in Chattanooga, certainly we believe [it] was linked to the homicides that occurred here in Clayton County," Register said.
Register said police wanted to locate the teenager because he has valuable information about the murders and because his life is in danger.
Investigators are trying to determine how the boy traveled to Chattanooga from Clayton County.
"Certainly we believe he has associations there in Chattanooga. We're still developing what those associations are and how he made his way from the Clayton County area to up there, and when," Register said.
"We are doing everything we can to bring the cowards who did this to justice," he added.