Richard Blong lured his teenage victims with a keyboard, authorities say.

On Facebook, he allegedly would engage them in conversation before asking them to send him pornographic images. In one case, the 27-year-old man is accused of inviting a 15-year-old girl to his Horsham home, where he allegedly sexually assaulted her.

Blong was charged last week with statutory sexual assault, involuntary deviate sexual intercourse, and related charges, the Montgomery County District Attorney's Office announced Thursday.

"Predators know the seemingly private nature of cell-phone use by children and teens and their various social-media accounts," Montgomery County District Attorney Kevin Steele said in a statement. "And predators seek to use those friendly platforms to take advantage of children."

Last month, authorities received a tip about possible child pornography or exploitation. The tip, they said, led them to investigate Blong. They said they found that he had coaxed three girls, between the ages of 14 and 17, to send him explicit photographs. Two of the girls lived outside the Philadelphia area and never met him in person, authorities said. A third girl went in August to Blong's home, where she was sexually assaulted, authorities said.

Blong failed to post his $100,000 cash bail and was being held Thursday at the Montgomery County Correctional Facility. He awaits a preliminary hearing Oct. 17.

No attorney for Blong was listed in court documents.