If you heard that the Oakland Police Department had "cleared," or closed, 60 percent of its felony sexual assault cases in 2016, you might think the police were doing a stellar job of solving rape cases and bringing perpetrators to justice. You would be mistaken.

In fact, according to an analysis of U.S. crime data obtained by ProPublica, Newsy, and Reveal from the Center for Investigative Reporting, almost seven out of eight cases in that year never even resulted in an arrest. Out of 341 total cases, the OPD only arrested the alleged perpetrator 13 percent of the time. Another 40 percent of cases were never solved. The remaining 47 percent of sexual assault cases reported to police were abandoned using something called "exceptional clearance."

Rape and other sexual crimes are admittedly difficult to investigate and prosecute. Cases typically pit the word of one person against another. Some physical evidence of rape can also be evidence of consensual sex. Victims sometimes decline to press charges rather than submit themselves to the ordeal of a prolonged investigation and trial. Prosecutors typically want to be sure they have rock-solid evidence when they take a case to court.

But none of those reasons appear to explain why Oakland police inexplicably terminated more than 250 felony sexual assault cases during the three years from 2014 to 2016.

The Federal Bureau of Investigation uses the phrase "exceptional clearance" to describe a set of reasons that police might stop working a case without making an arrest. U.S. law enforcement agencies report their success rates to the FBI every year as part of the agency's Uniform Crime Reporting program. Crimes that police take credit for resolving "by exceptional means" must meet the following four criteria:

1) The investigation has definitely identified the offender;

2) There is enough information and probable cause to arrest and charge the person;

3) Law enforcement knows the suspect's exact location;

4) For some reason beyond the control of law enforcement, the person cannot be arrested. Possible reasons include the accuser's desire not to press charges, the suspect's death or incarceration, or the district attorney's decision not to prosecute.

A case cannot be "cleared by exception" just because it may not be prosecutable, or because the victim doesn't want to pursue a case. The police investigation has to be thorough enough to yield the identity and whereabouts of a suspect and to substantiate probable cause for arrest. All four of the above criteria must be met for police to declare a case resolved in this manner.

From 2014 to 2016, Oakland police dismissed 143 alleged sexual assaults with the rationale "Victim Refused to Cooperate." Another 59 cases were closed because prosecutors reportedly declined to take the case to court. Two were presumably consensual but underage juvenile cases with a juvenile suspect not taken into custody.

But more than half of all the 2014-2016 cases that Oakland police claimed to have solved and stopped pursuing were neither dropped by the victim nor rejected by prosecutors. The data submitted to the FBI doesn't say why these cases were closed; six were tagged "Exceptional Clearance" and the other 247 were mysteriously labeled "Not Applicable." That's 253 separate Oakland sexual assault investigations in which the survivor wished to press charges and the police claimed to have cracked the case that were nonetheless abandoned for no apparent reason.

Why do Oakland police abandon so many cases of sexual assault even though they believe they have conclusively identified and located the attacker? Were those 253 suspects dead, or already in custody? That seems highly unlikely.

How, then, did something explicitly defined as an "exception" come to be used in nearly half of all reported sexual assaults cases in the city? Are Oakland police disinterested in such cases? Do they lack resources or personnel? Do their investigative techniques discourage rape victims from agreeing to press charges? And what about prosecutors? Do they fail to take good cases to court? Or is there a reasonable explanation for why only 13 percent of reported sexual assaults in Oakland result in an arrest when the national average is almost 22 percent? Do police in other cities care less about the quality of their evidence than officers in Oakland?

One thing is sure. You won't get the answer to these questions from the Oakland police.

In the absence of an official explanation for the high rate at which Oakland police claim success in rape cases even though they never arrest anyone, rape-victim advocates and sexual-assault prosecutors offered possible reasons for why Oakland police clear so many cases "by exceptional means."

When victims of sexual assault seek help at Highland Hospital, most are willing to talk to the police, said Kio Pak, the former program coordinator of the facility's Sexual Assault Response and Recovery Team, which works closely with victims. Their initial attitude, Pak said, is "I don't want other women to go through what I just went through." But such attitudes can change over the course of an investigation, which often moves slowly.