(CNN) A Texas sheriff's deputy is accused of a crime so heinous that his boss said it's "disgusting and infuriating."

Bexar County Deputy Jose Nunez, 47, allegedly sexually assaulted a 4-year-old girl -- then told the girl's mother, an undocumented immigrant, that she would be deported if she told authorities.

The 4-year-old girl is a relative of the suspect, Sheriff Javier Salazar said, but he didn't specify the relationship.

The sheriff said investigators have reason to believe there may be more child victims, and that the abuse may have taken place over months or years.

"The details of this are quite frankly heartbreaking, disturbing, disgusting and infuriating all at the same time," Salazar said.

'Super aggravated sexual assault' charge

Nunez now faces a charge of "super aggravated sexual assault of a child." If convicted, he could be sentenced to 25 years to life in prison, said Leslie Garza of the Bexar County district attorney's office.

Nunez's attorney, William Chumbley, did not immediately respond to CNN's request for comment Monday morning.

Super aggravated sexual assault of a child happens if the victim is under age 6, or if the victim is under age 14 and there are certain other factors, such as serious bodily injury or the presence of a deadly weapon.

Crying out for help

Authorities were tipped off to the case after the mother took her daughter to a fire station in the middle of the night, Salazar said. The girl, he said, had been crying for help.

Between 3 and 4 a.m. Sunday, Bexar County investigators arrested their colleague Nunez, who was off duty at the time.

Investigators are working to ensure the mother is given protected status pending the outcome of the case, Salazar said.

"I would urge anybody that may be undocumented that is a victim of a crime or a witness to a crime to please come forward and report it," he said.