Arellano's family has been spared having to view his body, but they were gathering dental records to confirm his identity.
Police said no one was in custody Sunday evening. They also said Arellano did not have a criminal history and was not affiliated with a gang.
Arellano's family is devastated and disappointed that, so far, no one has come forward with information about who murdered him and why. Relatives say they don't know of anyone who would have wanted to hurt him.
Some family members also say they wonder if it was Arellano's trusting nature that got him killed.
"I just want people to come forward in that area where it happened," said the victim's uncle Juan Tirado.
Tirado pleaded Sunday for help as he struggled with his nephew's murder. Relatives say they last saw Arellano alive Thursday night when he left his Southeast Side home allegedly to visit a girlfriend on the city's Southwest Side. They say he called for a ride home Friday night, but before they could pick him up, his girlfriend called saying Arellano was dead.
"Obviously, he was taken to a wrong neighborhood, and that ended up happening to him," Tirado told ABC7 Chicago.
Investigators returned to the neighborhood Sunday where the attack happened to gather possible evidence.
Police say Arellano has no criminal record and was not in a gang, but it appears he was targeted somehow.
Neighbors say the attacked happened around 1 a.m. Saturday between two houses. One woman who feared retaliation from gang members told ABC7 Chicago she heard the commotion and called 9-1-1.
A translator said: "She says she didn't know if it was a doll or a body. She just saw the body, and she just called police right away."
This fall, Arellano was a sophomore at Chicago Discovery Academy on the Bowen High School campus. He had taken special education classes, and was doing well in high school. However, relatives say he hadn't attended school recently because of problems with gangs threatening him. A CPS spokesperson confirmed that Arellano's last date of attendance was September 9, 2008 and added that school officials were told he was transferring to a different high school and were not aware of any harassment.
The boy's family claims they went to three different police stations before officers would take a missing persons report.
Chicago police responded in statement, which reads in part:
"We know police completed a missing persons report for the victim on May 1. And all details surrounding this incident are being thoroughly examined."
In the mean time, Alex Arellano's family is left with grief.
"People feel afraid to say something, but I urge people to say something and maybe help us," said Tirado.
Arellano's uncle also said the family had moved from California to Chicago when Alex was a baby, hoping for opportunity and a safer community for the children. He said the family was disappointed to find danger in their new life.
Even though Alex had not been attending class regularly, school officials said they still planned on sending crisis counselors to his high school Monday.