FONTANA - A man arrested on suspicion of molesting a longtime friend's daughter at her home Wednesday may have targeted multiple other children through his job as a part-time tutor, police said.

Anthony Castro, 40, was arrested at his La Habra home Wednesday evening, hours after the friend called authorities to report that he had walked in on Castro molesting his preteen daughter in the family's Fontana home, police said.

"He was over there visiting the friend when he was discovered doing more than visiting the friend," said Fontana police Sgt. Doug Wagner.

Castro had arrived at the house around 11 a.m. to see his friend, Wagner said. The man called police after Castro left the home, he said.

Detectives from the Fontana Police Department's Sex Crimes and Child Abuse Unit located Castro at his home in the 1500 block of La Habra Boulevard and arrested him at 7:16 p.m.

He was booked in lieu of $250,000 bail at West Valley Detention Center in Rancho Cucamonga.

Investigators believe Castro may have been molesting the girl on an ongoing basis, Wagner said.

Castro is believed to be a part-time tutor with clients primarily in the Orange and Los Angeles county areas, suggesting

he may have other victims, Wagner said. He was not a tutor to the Fontana girl, he said.

It is not known whether Castro worked through a tutoring agency or did the work on his own, Wagner said.

"The victim's father told us he thought the guy was a tutor," Wagner said. "When we asked (Castro) about it, he said, `I plead the fifth. I want a lawyer."'

A woman who answered the door at Castro's apartment would not comment, other than to say she did not believe Castro would have molested anyone.

Neighbors at the Tiki Gardens apartment complex expressed shock about the arrest.

"The most I've ever talked to him was out here in the back (at the complex) one day," said Mario Padilla, who lives nearby. "We talked about cars and stuff. ... He seemed like a nice enough guy."

Padilla said that he and his wife had checked California's online sex offenders database before moving into the complex to make sure their children would be safe from would-be predators.

He said it is scary that one may have been living there undetected all along. Police encouraged anyone with information on any other possible victims to notify law enforcement.

"Once we get his photo and this information out to the public, we think that several victims will come forward," Wagner said.

Anyone with information may call Fontana police at (909) 350-7700 or their local law-enforcement agency.

Staff writer Melissa Pinion-Whitt contributed to this report.