A sheriff’s deputy in San Antonio was charged with felony assault Sunday after being accused of repeatedly sexually assaulting a 4-year-old girl for at least several months, the authorities said, and threatening her mother with deportation if she reported him.

On Saturday night, the girl’s mother decided to take her daughter to a local fire station to report the deputy, Jose S. Nunez, after the girl “made an outcry” about the abuse, officials said at a news conference on Sunday. Mr. Nunez, 47, was arrested around 3:30 a.m. Sunday and charged with super aggravated sexual assault of a child, a first-degree felony because of the girl’s age, said Javier Salazar, the Bexar County sheriff.

“The details of the case are quite frankly heartbreaking, disturbing, disgusting and infuriating, all at the same time,” Sheriff Salazar said at the news conference.

Mr. Nunez was suspected of touching the girl inappropriately on numerous occasions at her home in recent months, Sheriff Salazar said, but he added that investigators were looking into whether the alleged abuse could have lasted several years. It caused the girl physical pain, including “some indication of minor injury,” the sheriff said.