(CNN)-- Two Maryland teenage girls were charged this week with assaulting an autistic boy and recording him on their cell phones performing sex acts, including some with animals, the St. Mary County Sheriff's Department said Wednesday.
Authorities said the two girls, ages 17 and 15, admitted to police that they assaulted the boy, 16, with a knife on "multiple occasions."
"His parents refer to him as (having) been diagnosed autistic," said Sgt. Cara Grumbles, a spokeswoman for the sheriff's office. "Some of the sexual acts involved animals. It's awful."
A news release from the department said the girls also kicked the boy in the groin and dragged him by his hair. They also got him to walk onto a partially frozen pond. When the boy fell through several times, they made him get out each time by himself, police said.
Detectives found videos on the girls' cell phones.
Prosecutors charged the 17-year-old, Lauren A. Bush, as an adult. The 15-year-old was charged as a juvenile. Police didn't release her name.
Each was charged with two counts of first-degree assault, two counts of second-degree assault, child pornography and false imprisonment.
CNN's John Newsome contributed to this report.