In an extraordinary turn of events in an equally extraordinary murder case, prosecutors on Friday dropped charges against two boys, ages 7 and 8, who had been accused of killing an 11-year-old girl by hitting her with rocks and suffocating her with her panties so they could steal her bicycle.

The case was dismissed after a laboratory analysis found semen on the girl's panties. Cook County State's Attorney Richard Devine said medical experts told prosecutors that "the possibility of semen coming from boys this age is at best remote."The case against the boys brought national attention because they were believed to have been the youngest children charged with murder as younger and younger children are being charged with serious crimes.

Prosecutors and police said Friday that they were now confronted with reconciling the physical evidence with the statements the boys made. Police refused to rule out the possibility that the boys were still involved. Authorities said they had not done genetic testing to date and declined to say whether they planned any in the future.

Chicago police had said the boys confessed to the July 27 killing of Ryan Harris, a fifth-grade honor student from a Chicago suburb who was spending the summer with a godmother in the impoverished Englewood neighborhood, where the boys lived.

They said the boys struck Ryan with a rock, causing her to fall off her bicycle and knocking her unconscious. Then, police said, the younger boy told them he put leaves in Ryan's nostrils and stuffed her panties in her mouth.

Faced with such apparently depraved actions by children so young, commentators took to the airwaves and opinion pages bemoaned the circumstances that could have led children to commit such a crime.

But Friday's dismissal is likely to raise a whole new debate - about the ability of juvenile systems in Chicago and elsewhere to be fair and accurate in their investigation of society's youngest accused criminals.

From the outset, the Ryan Harris case raised fundamental questions about the way to handle young children charged with crimes. Some experts questioned whether children so young could waive their right to a lawyer or their right to remain silent.

The boys' defense lawyers had complained about the way their clients were interrogated, saying that because their parents were not allowed in the room and no juvenile advocate was present, police could have coerced or teased the statements out of the boys.

Friday, defense lawyers for the boys were already calling for significant changes in juvenile proceedings in Chicago, the home of the nation's first juvenile court. Their demands included a requirement that police officers videotape confessions from children.

"I think there was a rush to judgment," said Elizabeth Tarzia, a public defender for the 7-year-old. "There were policies and procedures that were not in place in juvenile court that led to this happening."

Friday's dismissal also provoked an explosion of reactions from the boys' families and neighbors.

"An apology is in order from everyone who convicted my child of something that it should have obvious he had nothing to do with," said the 8-year-old's father.