"After he broke up the fight, he started having trouble with the suspect," said Lt. Christopher Cook of the Arlington Police Department, per NBC News.
Lt. Cook said video surveillance captured the suspect — described as aged between 13 and 15 years old — at the apartment complex, where he also lived.
"He pulls out a handgun from the rear part of his pants he was wearing, points it at the victim and fires one round," he told reporters. "We did recover one spent shell casing. It's a .40 caliber semi-automatic handgun."
Samuel's distraught mother Madison also told reporters she had warned her son to stay away from the suspect following the bullying incident.
Police have yet to recover the murder weapon, or deduce where the suspect got it from.
“That's something we want answers for. We are sick and tired of children in our community coming into contact with firearms and possessing them and using them in these types of manners," Lt. Cook said.
"If an adult has provided this firearm, you can certainly bet that we're going to pursue them vigorously because firearms and kids – they just don't mix."
The suspect — whose identity has not been revealed because of his age — faces a murder charge; juvenile court will decide if he will be tried as an adult or child.
Samuel's cousin Courtney Fletcher set up a GoFundMe to raise money for her aunt Madison, who lost her only son.
"Madison and Sam had a stronger bond then you could ever imagine, it has always been the two of them, the best of friends," she wrote. "Madison raised Sam to be the most respectful, kind hearted person you've ever met."