Benjamin, a freshman at Valdosta State University, was found Nov. 18 on a dormitory study room couch. On Thursday, fellow Valdosta State student Darien Meheux, 18, turned himself in after learning there was a warrant for his arrest, says Steven Turner, an agent with the Georgia Bureau of Investigation.
"Right now he's the sole suspect," Turner tells PEOPLE. "They had dated at one time. The status of their relationship back in November is not necessarily clear whether they were dating or not."
Frustrated by the lack of answers and the way they'd learned about Benjamin's death, her family hired a private investigator, Robin Martinelli, but believed all along that Meheux might be involved, says Martinelli.
Benjamin and Meheux both were from Lawrenceville, about 30 miles northeast of Atlanta.
Authorities earlier had said the cause of death was asphyxiation, and that Benjamin may have been dead for several hours before her body was discovered in the shared common area of Georgia Hall on the 10,000-student campus in Valdosta, Ga.
Benjamin's mother Judith Brogdon and stepfather James Jackson were notified of her death when a friend forwarded a Facebook post about the discovery.
Officials then told them their daughter died from natural causes, until the medical examiner changed the investigation within 24 hours to homicide.
"To find out it was a homicide and that somebody actually murdered our daughter changed everything," Jackson tells the Atlanta Journal-Constitution. "It was like hearing the news all over again."