JACKSONVILLE, Fla. -- A registered sex offender has been arrested after he took a photo under a girl's skirt in a public library.

William Thomas, 28, was seen by the victim's brother, who told their mom. It happened on Tuesday at the Pablo Creek Library located on Beach Boulevard.

The brother of the victim witnessed Thomas squatting behind his sister with a black cell phone in his hand, and he saw Thomas put the cell phone under her skirt.

A spokesperson for Jacksonville Public Libraries calls it a very unfortunate situation. He said he commends the family of the victim for immediately alerting staff who called police.

Deborah Ward, who frequents the Jacksonville library said she saw Thomas on Wednesday.

"He was roaming about not particularly getting to a destination like the rest of us," she said. "A lot of children come here, we came too close, it was too close to having a really bad situation."

Thomas has a prior conviction for lewd/lascivious conduct on a child in 2009 for allegedly threatening to kill a girl and touching her during a burglary. For that, he was required to register as a sex offender.

In 2007, Thomas was arrested for grabbing a girl's buttocks at Mandarin High School, but the case was dropped to a simple battery, the State Attorney's Office said.

Williams lives less than a half-mile from an elementary school. According to JSO, sexual offenders are free to live where the want, unlike sexual predators.

The Jacksonville Sheriff's Office said that there may be additional victims and if anyone knows anything, they should contact JSO at 904-630-0500.