CHICAGO -- Authorities said a teenager accused of abducting two young children in Chicago intended to perform a demonic ritual.

They said he was trying to get a former girlfriend back.

Prosecutors said 18-year-old David Rodriguez and a 15-year-old companion snatched Erika Castillo, 6, and her older brother, Oscar, outside a library on Friday and planned to carve a pentagram in the girl's chest.

Police found the girl unharmed at Rodriguez's home after the girl's brother, who had been released, told people on the street about the abduction. Rodriguez apparently needed only the girl, so he released her 8-year-old brother, Cook County Assistant State's Attorney Tracy Senica said.

Police said Rodriguez made up an errand to get rid of Oscar and took Erika into his home.

"He told me to take a paper to the doctor," Oscar said.

Police arrested Rodriguez when he returned with candles and incense.

Rodriguez is still jailed. His grandfather, Julian Rodriguez, called the accusations "ridiculous" and said that his grandson is a "good kid." He also said he saw his grandson with the girl.

"I said, 'David, what are you doing with that little girl, do you know her?' And he said, 'Yeah, I used to baby sit her. She's lost, grandpa,'" Julian Rodriguez said.

Julian Rodriguez said his grandson has medical problems, and admitted that he recently left his church and turned to devil worship.

"I don't believe in it, but he does," Julian Rodriguez said.

Rodriguez was being held on a $500,000 bond and was ordered to undergo a mental evaluation.

Rodriguez was charged with two counts of aggravated kidnapping, police spokesman John Mirabelli said.