Two people have now been charged in the case. One of them appeared in court on Thursday.
Jovanny Martinez, 15, -- who was arrested on a weapons charge the same night Arellano was murdered-- was denied bail at a court hearing on Thursday. He is accused of shooting Arellano in the head and police say a ballistics test shows Martinez's gun was used in the murder.
"The victim's blood was found on his pants and the gun in his possession was the firearm that killed victim," said John Dillon, assistant state's attorney.
Edgar Silva, 18, will appear in bond court on Friday. Silva faces first-degree murder charges in the case.
During the hearing, prosecutors said Arellano was beaten, then shot in the head and then set on fire.
According to prosecutors, Arellano was on his way to a party with some girlfriends on May 1 when he was confronted by two men on bicycles. They asked him what he 'was about,' which referred to his gang affiliation, prosecutors said.
"The victim says he wasn't about anything, he wasn't in a gang.The individuals rode off on bike. Within a very short period of time a Mustang pulled up with known members of Latin Kings street gang (they) got out with baseball bats and beat the victim," said Dillon.
Police said Arellano got away, but then was run down by the driver of the Mustang. Arellano escaped for a second time as he fled into the yard of a vacant home, but the alleged gang members caught up with Arellano a third time.
"While in the backyard the beating continued. (The) victim was shot one time in the head. The offenders left," said Dillon.
Hours later, Arellano was set on fire.
The victim's family can't believe a teenager is responsible for such a brutal crime. Martinez's attorney calls his client 'just a little boy.'
"Who thinks of doing this? Especially at that age? Who thinks of doing this to another child," said Juan Tirado, victim's uncle.
"It's obviously quite concerning. It's a serious charge. But (he's) confident that ultimately he'll be found not guilty," said Tim Biasiello, suspect's attorney.
Prosecutors would not say if Martinez was involved with the baseball bat beating prior to the shooting.
Police say Arellano was in the wrong neighborhood at the wrong time. Refusing to acknowledge the Latin Kings street gang is the only motive prosecutors have released publically. Arellano's family has said he was harrassed by gangs before the killing.