Horacio Alvarado, 32, allegedly impregnated his step-daughter when she was just 15 and then sought a paternity test in Milwaukee, believing it would keep him in the US

An illegal immigrant in Milwaukee is accused of fathering his teenage step-daughter's child in a misguided attempt to remain in the US.

Horacio Alvarado, 32, allegedly impregnated the girl when she was 15 and then sought a paternity test, believing it would keep him in the country.

The girl gave birth to the child back in 2012 but police only uncovered a lengthy history of alleged assaults last month when they started investigating a new sexual assault claim made by the victim.

The step-daughter, now aged 22, was being interviewed over the recent sex assault allegation when she told police Alvarado had repeatedly abused her as a teenager, according to a criminal complaint obtained by the Milwaukee Journal Sentinel.

She told authorities Alvarado raped her on average three times a week from when she was 14 until she was 18 years old. The alleged abuse continued even after she gave birth to his child.

The woman told police that Alvarado allegedly assaulted her when her mother, Linda Alvarado, was at work or sleeping late at night.

The Milwaukee County Circuit Court established Alvarado as the child's legal father in 2015 after he sought a paternity test following the birth in 2012

She claims her mother once walked into the bedroom while Alvarado was raping her, but she left the bedroom without trying to stop him.

The woman told authorities she kept the identity of her baby's father a secret from everyone except her mother, who she told shortly after the child was born.

The Milwaukee County Circuit Court established Alvarado as the child's legal father in 2015.

The criminal complaint states that Alvarado believed a positive paternity test would grant him permission to remain in the country.

Alvarado was arrested and charged last month with first-degree sexual assault, repeated sexual assault of the same child and incest with a child by a stepparent.

He is being held at Milwaukee County Jail on $20,000 bail.

The girl's mother, Linda Alvarado, has also been charged with misdemeanor child neglect. She was released on a $500 bond.