A review of more than 1,000 sexual assault cases in Minnesota reveals chronic errors and investigative failings by the state's largest law enforcement agencies. In almost a quarter of the cases, police never assigned an investigator. In about one-third of them, the investigator never interviewed the victim. In half the cases, police failed to interview potential witnesses.About 75 percent of the cases, including violent rapes by strangers, were never forwarded to prosecutors for criminal charges. Overall, fewer than one in 10 reported sexual assaults produced a conviction.