Brooklyn district attorney Charles Hynes has refused a freedom of information request for details of child sex abuse prosecutions in the Orthodox Jewish community, amid questions over the veracity of his arrest figures.

The refusal coincides with the collapse of his most vaunted prosecution after appellate court judges ruled this week for a mistrial in the case of Baruch Lebovits, sentenced in 2010 to a maximum 32-year jail term.

Lebovits was convicted in March 2010 of abusing a 16-year-old boy. The DA's office said it is prepared to retry the case. But victims' advocates say his mistrial is a major setback in their efforts to encourage other child sex victims to risk community censure and come forward. Advocate Joel Engelman said the decision was "extremely demoralising".

Last month the Guardian reported that the DA was facing accusations that he has failed to tackle up the cover-up of abuse in Brooklyn's Orthodox community – the largest Orthodox community in the world outside Israel. Rabbinic leaders are accused of hampering efforts to uncover abuse, and many victims face intimidation and threats to prevent them pressing charges.

The Guardian and the Jewish Forward paper had submitted freedom of information (FOI) requests for details of cases. But this week Hynes's office informed the Forward that it was denying its request, because the close-knit nature of the Orthodox community meant disclosing the names of defendants would reveal the identities of the victims. The DA's spokesman, Jerry Schmetterer, confirmed that the Guardian's request would also be refused on the same grounds.

The statement refusing the FOI request marks the first admission by Hynes that suspected child abusers from the Orthodox Jewish community receive exceptional treatment.

Hynes's policy clarification follows recent claims to have radically increased prosecutions through his Kol Tzedek outreach effort. Kol Tzedek, which features a victims' hotline staffed by a culturally sensitive social worker, aims to encourage reporting of sexual assaults and to break the cover-up of child sex abuse by the Orthodox Jewish community.

The DA credits the initiative, which began in April 2009, with a surge in arrests. In January, the DA said the tally now stood at "over 90" arrests, a figure confirmed again this week by his spokesman.

Of the 38 cases closed by November last year, 23 had concluded in guilty pleas and six in trial convictions with nine dismissals and acquittals, Schmetterer said. There were 14 prison convictions. But at least nine of the arrests ascribed to Kol Tzedek were in fact made before April 2009.

The Guardian has learned of five: Stefan Colmer, extradited from Israel in 2008, Yona Weinberg, indicted in 2008, Emanuel Yegutkin, arrested in January 2009, Moshe Spitzer, arrested in March 2009, and Baruch Lebovits, whose landmark conviction was overturned this week and who was indicted in 2008. The DA's spokesperson confirmed in writing last month that all five men were being counted as Kol Tzedek arrests.

The Jewish Week knows of a further four arrests inaccurately credited to Kol Tzedek: one convicted of attempting to kidnap an Hispanic girl, arrested in 2007; another in mental health facility and unfit to proceed, arrested 2008; a third who violated his probation and was resentenced in 2009, first convicted in 2002; and Yehuda Kolko, whose case in 2008 saw him plead guilty to misdemeanor child endangerment, not a sex crime. Kolko is currently being prosecuted for criminal contempt.

Schmetterer refused to answer questions from the Guardian. "I have nothing to say to you," he said. "You have the (FOI) letter. I have nothing to add."

The FOI response – set out in a letter signed by assistant district attorney Morgan Dennehy – appears to explain an apparent misinterpretation of a civil rights statute previously used by the DA to explain why case information was being withheld from public scrutiny.

Earlier responses from the DA's office have cited New York Civil Rights Law 50B, which protects the privacy of sex crime victims. 50B prohibits the release of documents that could identify a victim. It does not, however, restrict the release of case files in which the victims' names or other revealing information has been redacted, said Martin Guggenheim, professor of clinical law at NYU. "To my knowledge," Guggenheim said, "the law permits the disclosure of reports so long as the public official does so in a manner that ensures that the identity of the victim cannot be obtained from the report."

Hynes's argument – that the "unique" circumstances in the Orthodox community prohibits release of any and all information, since to name the defendant is to name the victim – apparently explains his over-stringent response to 50B.

Dennehy's letter goes on to say that sex crime victims from the Orthodox community, if identified, face unique challenges that will make them likely to withdraw co-operation "making the prosecution of the defendants extremely difficult". It would also prevent victims of unreported crimes coming forward in the future if they feared their names would become known. Under New York's FOI law, documents which could "interfere with law enforcement investigations or judicial proceedings" are exempt from disclosure.

Roger Canaff, a former sex crimes prosecutor in the Bronx, said he thought the reason for the DA's silence was justified.

"There are some situations where releasing suspect information could lead to unfairly revealing victim information," Canaff said. "I can imagine a situation where that could happen. So for the sake of the victims I think it's OK to make an exception.

"I haven't dealt with it myself, or seen it anywhere else, but I can understand an office being concerned with that. I can't say whether that's what is going on here, or something else, but I think it's a plausible explanation."

Community activist Isaac Schonfeld, an Orthodox Jew from Borough Park, said the DA's position was illogical because if the everybody in the community really did know everybody else's business, the victims' identities woul already be known. "I don't find the argument compelling," Schonfeld said.

"We have this wonderful compassion in our community. Rather than channelling that into covering up [child abuse], we should be using it to develop a more comprehensive understanding of the problem, and giving the victims tools to deal with it rather than just sweeping it under the rug."

"The DA should be challenging the community to bring these issues into the open and to bring the perpetrators into public view," he added.