Online, he allegedly chatted with a girl under the age of 13. Last month they met for the first time.
"The suspect actually picked the victim up as she was getting out of her middle school. He took her back to the Back Bay area of Newport Beach, there's trails and greenbelt area back there where apparently the crime occurred," said Orange County Sheriff's Lt. Steve Doan.
Investigators suspect Robertson had sex with the girl two more times at his apartment in Newport Beach up until November 1. Each time after, he would drop her off at her home in Ladera Ranch.
Robertson was arrested Tuesday night after the girl's mother intercepted his text message as he tried to arrange a fourth rendezvous with the girl.
Orange County Sheriff's deputies were waiting at a greenbelt area in Ladera Ranch.
"Anybody who would do this kind of thing to a child is a dangerous individual," said Doan.
Robertson told investigators he works as a computer programmer. At least one neighbor who did not want to be identified said Robertson was tutoring her daughter in math. The neighbor was shocked by his arrest.
No one answered at Robertson's apartment Wednesday. He shares it with two other adults. Hours beforehand, investigators seized potential evidence, including Robertson's computer.
"Our big concern is the fact that he was out on a social website. So given that he was able to communicate with this child, we are fearful there might be other victims," said Doan.
Authorities would not name the site, concerned that it could identify the victim.
If you have information related to the suspect, you're asked to call the Orange County Sheriff's Dept. at (800) TIP-OCSD.