D.C. police failed to investigate at least 170 sexual assaults over a three-year period, a human rights group alleges in an analysis that the city’s police chief called fundamentally flawed and based on a misunderstanding of department data.

A report by Human Rights Watch, due to be released next week, concludes that one-third of 480 sexual assault cases reported to police by women treated at MedStar Washington Hospital Center from October 2008 to September 2011 were not documented in initial police reports or assigned a file number used for tracking.

Sara Darehshori, the report’s author, said she concluded that those cases, and perhaps many more, were closed without further review. “It’s the incident report that matters in terms of whether there’s an investigation,” Darehshori said.

D.C. Police Chief Cathy L. Lanier, in a letter to Darehshori, said the Human Rights Watch staff mischaracterized data gathered during a year-long examination of public and private files. She accused the group of “selectively using the facts” while “continuing to take anecdotal information out of context to suggest larger problems.”

In the same letter, Lanier said police have made significant changes to the way reports of sexual assault are taken and classified and have improved training and added staff to the sexual assault unit in recent months.

D.C. Police Chief Cathy Lanier speaks at a press conference on Jan. 3. A human rights group alleges D.C. police failed to investigate at least 170 rapes and sexual assaults over a three-year period. Lanier calls the conclusion fundamentally flawed. (Robert MacPherson/AFP/Getty Images)

The number of sexual assaults in the District rose 51 percent from 2011 to 2012.

The dispute between Human Rights Watch and D.C. police has lasted months and has been conducted publicly, including in court. Lanier’s office posted the latest exchange of letters on the department’s Internet site, in effect publishing some of the group’s findings and the chief’s rebuttal ahead of the report’s release.

Lanier declined to comment Thursday, referring to her letters. Darehshori declined to provide a copy of the report before its scheduled release next Thursday.

Human Rights Watch says the full report will be about 200 pages long and include narratives from women who said officers didn’t take their claims seriously or properly investigate their allegations. Some accounts, summarized in the exchange of letters, allege that police dismissed victims who had been drinking or who couldn’t recall precise details.

One rape case, reported in 2010, was closed and then reopened nine months later, the group said.

In her letter, Lanier called accusations that D.C. officers treated victims poorly “a cheap shot.” She criticized Human Rights Watch for not providing her an advance copy of the report and accused it of being “secretive and opaque” after she allowed it “unprecedented and complete” access to files.

Darehshori said Human Rights Watch obtained access to the files only after filing a lawsuit.

Human Rights Watch studied reports from Washington Hospital Center, the only District medical facility that performs forensic exams of sexual assault victims. The group did not include cases in which the victim declined to talk to police or did not want a report taken.

Darehshori said in a letter to Lanier that the hospital had 480 files of women who told D.C. police that they had been sexually assaulted. She said the Human Rights Watch team could not find police reports in 170 of the cases, or about 35 percent.

In 34 instances, Darehshori said, police reports on sexual assaults, with dates matching reports from the hospital files, were classified as “miscellaneous” or were given other designations that signify a lower priority.

In her response, Lanier called the conclusion that those cases were not investigated “untrue and shows a complete misunderstanding of the classification of reports at the time.” The chief said such notations were typically used when “the preliminary investigation did not reveal enough information” to substantiate a crime. She said the classification was subject to change.

Lanier agreed with Human Rights Watch’s recommendations regarding the reporting of sexual assaults. Now, Lanier said, “public reports are taken on all cases, and they are classified as either a sexual allegation or a sexual abuse case.”

In her letter, Lanier says Human Rights Watch researchers — who examined not only public police reports but also an internal database containing additional reports of sexual abuse — should have been able to locate all the cases it was seeking.

In an interview Thursday, Darehshori said the group is confident of its conclusion, even after examining the database.

Lanier said in her letter that the group was given 1,080 incident reports regarding sex abuse. She also said sexual assault cases are among the most difficult to investigate and prove in court. Nationally, she said, two-thirds of sexual assault suspects are known to the victims, there typically are no other witnesses, and “consent is extremely difficult to prove or disprove.”

Lanier said last summer that she asked the Justice Department to investigate the sexual assault unit. A Justice spokeswoman said Thursday that department attorneys are reviewing the request.

Sexual assault statistics are often controversial. Advocates say the figures can easily be manipulated, with cases complicated further by social stigma and the reluctance of many victims to trust the police.