The boys had been charged with the juvenile equivalent of first-degree murder

CHICAGO (CNN) -- Chicago prosecutors on Friday dropped their case against two young boys accused of killing an 11-year-old girl in July.

A crime laboratory report issued to prosecutors Friday shows semen was present in the girl's underwear.

Experts say it's highly unlikely that 7- and 8-year-old boys could produce semen.

Eleven-year-old Ryan Harris was reported missing July 27 and found dead the next day on Chicago's South Side. She was beaten, suffocated and sexually molested.

Two neighborhood boys, ages 7 and 8, were charged as juveniles with first-degree murder after telling police they threw rocks at the girl. They were the youngest murder defendants in Chicago history.

Neighbors of the boys were outraged over their arrests, some expressing disbelief children of that age could be capable of such a brutal crime. Attorneys for the boys claimed this week that police framed them.

The boys had been held under house arrest monitored via electronic restraints.