Lincoln. Neb. ? A 22-year-old man accused of impregnating then marrying a 14-year-old girl pleaded not guilty Tuesday to raping her.

Matthew Koso was arraigned in Richardson County District Court, where he is charged with first-degree sexual assault, punishable by up to 50 years in prison.

The girl, Crystal Koso, gave birth Aug. 24 to a 7-pound, 1-ounce girl, Samara.

Nebraska requires people to be at least 17 before they can marry.

But after the girl became pregnant, her mother gave permission in May for Koso to take her daughter to Kansas, which allows younger minors to get married with parental consent.

Kansas Gov. Kathleen Sebelius recently said she was asking legislators to establish a minimum age for marriage. She said the age should be 16, 17 or 18, “so we don’t, after the fact, cover up abuse of children.”

Nebraska Atty. Gen. Jon Bruning said again Tuesday that additional charges against Koso involving other young girls are possible.

“Our investigation’s continuing,” he said. “Obviously, it is dependent on very young girls telling us that they had sexual relations with Mr. Koso, which is not an easy fact to elicit from a very young girl.”

Koso’s trial date was set for Oct. 13.

Bruning acknowledged that he has received a “significant” amount of mail from the public saying he should leave the couple alone.

“But this is not a job we do to win popularity points,” Bruning said. “I was elected to enforce the law. And anytime a grown man has sex with a child, it’s not a close call for me.”

A Falls City Police investigator testified earlier that the case began last May when the state Health and Human Services System asked the department to look into a report that the girl might be pregnant.

Bruning said that Koso is a friend of the girl’s half brother and began a relationship with her when she was 12.

Koso’s attorney, Willis Yoesel, did not immediately return a call to his office seeking comment.