The suspected crime took place on January 1 this year.
With the naked sex, the man must have performed intercourse-like acts against the child, according to a witness who saw parts or the entire course of events.
The suspect was arrested a few days later and has been in custody ever since.
According to the lawsuit, semen was found on the child and the man's DNA was found in the crotch of the body that the child was wearing at the time.
The man, who denies the crime, is now suspected of aggravated rape of a child.