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AYER — In what a prosecutor described as an “extremely disturbing case,” a 21-year-old Ayer man is being held on $50,000 cash bail after he is accused of raping a 14-month-old child.

Christopher D. Banks, of 75 Groton School Road, Unit V6, pleaded not guilty yesterday to rape of a child with force and aggravated rape. He was ordered held on $50,000 cash bail or $500,000 surety. A probable-cause hearing is scheduled for May 11.

At his arraignment in Ayer District Court, Banks, who was still dressed in his work shirt with his name on it, repeatedly looked over at his mother, father and brother who were seated in the courtroom.

Prosecutor Felicia Sullivan said that at 10 p.m. Wednesday, Ayer police received a report of a man who had sexually assaulted a child.

Sullivan said the child had been in Banks’ care that day until she was picked up by a caretaker. The caretaker went to change the baby’s diaper and noticed something wrong, Sullivan said. The child was rushed to a doctor, who said the child’s injuries indicated a sexual assault. The child was taken to the hospital.

About 50 minutes after police were notified of the alleged rape, Littleton police contacted Ayer police to report that Banks had turned himself in, Sullivan said.

Banks allegedly confessed to police that he had raped the crying child, put her diaper back on, handed her over to her caretaker, then went to work, Sullivan said.

Sullivan asked for the high bail, saying Banks “took advantage of an innocent child.”

She added that Banks is a threat to his own family: He is married, with an 18-month-old child and a baby on the way.

If convicted of aggravated rape charge, Banks faces a minimum mandatory sentence of 15 years in prison, up to life behind bars, Sullivan said.

Defense attorney Roland Milliard asked Judge Michael Brooks to set a $5,000 cash bail with conditions. Millard said Banks has no prior record, gave police a statement, and has indicated his “remorse” and “regret.”

Milliard said Banks turned himself in and is willing to “submit himself to the system and see what happens.”

Milliard told the judge that if he would release Banks, a maintenance man, on low bail, he would agree to live with family members in Medford and stay away from his child.

If Banks posts bail, Brooks ordered that Banks is under house arrest at the relative’s home and must wear a GPS ankle-monitoring system. He must not have contact with any child under 18, including his own, and stay away from the victim and her family’s home, school or day care.

Follow Lisa Redmond at www.twitter.com/lredmond13.