"Very quickly the conversations turned to sex," Amormino said.
Riddle told the girl he was 24. Amormino said he bought her a cellphone and sent her sexual text messages and mentioned hotel rooms in his conversations with her.
He instructed the girl to "touch herself" and take naked pictures of herself and send them to him using the cellphone, Deputy Dist. Atty. Beth Costello said.
"She definitely said that she was 14 years old with no sexual experience at all," Costello said.
The girl, who recently turned 15, had told her friends and adults about her contact with the man, and the authorities were alerted, Amormino said.
Investigators arranged a telephone conversation between Riddle and the girl, and set up the meeting at the San Clemente hotel, authorities said. There Riddle was arrested by sheriff's deputies, officials said.
Although Riddle and the girl never met in person, his alleged actions are considered illegal because he instructed her to touch her body at his request, Costello said.
He is charged with lewd acts with a minor and using a minor for sex acts, officials said. He is being held on $100,000 bail at Orange County Jail and is scheduled to be in court Tuesday.
Investigators were searching Riddle's computer Wednesday for any further evidence.
"If he's online and he's seeking out these young kids and he's pretending to be a young stud ... chances are he's done this before," Costello said.
The Federal Bureau of Investigation runs a task force -- Innocent Images National Initiative -- that specifically patrols the Internet for online predators exploiting children.
FBI agents and other law enforcement officers go online undercover using fictitious screen names and engaging in real-time chat or e-mail conversations with subjects to obtain evidence of criminal activity.
Costa Mesa and Newport Beach police do not actively monitor online sites to enforce rules about contact between adults and minors, officials said.
Officials from both police departments said they aren't aware of any current cases of online sexual predators contacting minors in Newport Beach or Costa Mesa.