Qutravius Palmer, 22, turned himself in to police on Jan. 1 and is charged with felony murder and armed robbery.
He was taken to the DeKalb County Jail.
Palmer's arrest follows that of a 14-year-old, whom investigators charged on the same counts.
Arnold, a 21-year-old Army Reservist and art student at the Savannah College of Art and Design, was shot in the head during a robbery as he, his girlfriend and a third friend walked along a bike trail between the 200 blocks of Warren and Rogers streets in Northeast Atlanta.
The investigation continues, but it's not clear if any other arrests are forthcoming. The robbery itself involved two gunmen, but more people may have been involved.
It was a point that Capt. Paul Guerrucci, who runs APD's homicide unit, alluded to last week during a news conference.
He said investigators are looking for any and all people who may have been involved, not just the suspected gunmen, for prosecution.
"Did you plan it, were you involved with them, did you go with them, or did you take any of the proceeds from these acts," Guerrucci said. "We will actively seek you for your role."
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