An off-duty Bexar County Sheriff's Department deputy has been arrested on a felony charge involving sexual assault of a child, reports CBS San Antonio affiliate KENS-TV. Jose Nunez, 47, is a detention deputy who's been with the department ten years.

Sheriff Javier Salazar says the victim's mother is an undocumented immigrant, and the suspect took advantage of the mother's fear of deportation.

Nunez is charged with super aggravated sexual assault -- a charge applied when the victim is less than six years old.

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Salazar says the victim, who is 4, cried out to her mother Saturday night.

He said the mother decided to go to a local fire station to report the abuse.

Investigators quickly stepped in and arrested Nunez, around 3 a.m. Sunday.

Salazar said Nunez touched the girl's genitals and hurt her.

Investigators believe the abuse could go back months or possibly years.

"When one person or persons individually demonstrate like they did in this case that they're not worthy to be part of this agency, then we're going to, quite frankly, get them out of here. Cut them out like cancer. And that's what's going to happen in this case," said Salazar.

Nunez has been placed on administrative leave pending an investigation. Internal affairs will also be conducting a separate, but concurrent

Salazar said he encourages undocumented immigrants to continue to report crimes. He said there could be more victims in Nunez's case.

"(We ask) victims of a crime or a witness to a crime to please come forward and report it. Just like we're doing in this case, we're filling out paperwork with this witness in question to make sure that she's given protected status pending the outcome of this case," Salazar said.