Nebraska ? Two 13-year-old boys videotaped themselves sexually assaulting a 5-year-old girl and recorded sexual contact between her and two other young children, police said.

Police said the mother of one of the victims found the videotape Jan. 18. The assault was believed to have occurred within the past year at one of the boys’ homes, about a block away from an elementary school.

Police believe all the children are related.

The tape shows the boys assaulting the girl with an object, and she is also seen having sexual contact with another 5-year-old girl and a 3-year-old boy, police said.

“What they were trying to do is basically simulate sex acts,” said Officer Katherine Finnell.

Finnell said police did not know where the parents were during the assault and taping.

The 13-year-olds were taken to Lancaster County Juvenile Detention Center, Finnell said.

Both boys were on juvenile probation when the tape was found. One was sentenced in December for burglary and criminal mischief. The other boy had been on probation for burglary and assault since August.