Manhattan District Attorney Cyrus Vance Jr. requested a special prosecutor last year to investigate an intern's accusation that she was sexually assaulted by a high-ranking member of his office. View Full Caption Mario Tama/Getty Images

NEW YORK CITY — A female intern in the Manhattan District Attorney’s Office claimed last year that a high-ranking employee in the office sexually assaulted her, but her accusations were not investigated because she declined to cooperate with a special prosecutor, DNAinfo New York has learned.

Staffers working for Manhattan DA Cyrus Vance Jr. initially tried to look into the intern’s claims. But when she refused to discuss any of the allegations, Vance asked the administrative judge for New York City courts to assign a special prosecutor to investigate the matter “to avoid any appearance of impropriety.”

“I am informed that a former legal intern in this office has accused an employee of this office of sexually assaulting her,” Vance said in a March 31 affirmation in support of his application for the special prosecutor.

“The former intern will not divulge her identity, the identity of her assailant or any details of the sexual assault to anyone in my office, other than saying that her assailant is ‘high-ranking.’”

The DA’s office learned of the accusations through a friend of the intern, according to a source. Vance said in his affirmation that without the intern’s cooperation, he believed a special prosecutor was appropriate.

"Since this person will not cooperate with my office and will not divulge the name or position of her assailant, we are unable to investigate her claims," Vance said.

“Therefore, the interests of justice dictate that an application for recusal and appointment of a special district attorney be submitted to this court. This application is made to avoid any appearance of impropriety and to allow these serious allegations to be investigated."

A judge granted Vance’s request on April 4, assigning the probe to the Bronx District Attorney’s Office.

But Bronx DA spokeswoman Terry Raskyn said her office never investigated the accusations because the intern “did not wish to share any information with us” and would not reveal her or her alleged attacker’s identity.

Joan Vollero, a spokeswoman for the Manhattan DA’s Office, did not say how much time passed between when her office learned of the allegations and when Vance asked for the special prosecutor. She also would not say why her office tried to look into the allegations before requesting a special prosecutor.

“As the affirmation indicates, the complainant refused to identify herself or the employee in question, or provide details about the purported incident,” Vollero said. “The investigation has been referred to the Bronx District Attorney’s office and any further questions should be directed there.”

The Bronx DA’s office also didn’t respond to a question asking how long Vance's office waited before requesting a special prosecutor. It would also not say what steps, if any, were taken to try to obtain her cooperation.

Both the Manhattan DA's office and the Bronx DA's office would also not say whether they alerted the NYPD to the accusations or tried to refer the accuser to support services.

Eugene O'Donnell, a professor at the John Jay College of Criminal Justice, said that accusations like these require investigators to use all the tools at their disposal to find out what happened.

"This is not an ordinary run-of-the-mill accusation, obviously," said O'Donnell, a former NYPD officer and prosecutor in the Queens and Brooklyn district attorneys' offices.

"Has the DA pulled out all the stops to secure cooperation and physical evidence? Has NYPD been asked for assistance beyond the prosecutor's office? This calls for NYPD's A-team.

"Has thought been given to compelling the witness' testimony? Has the victim been referred to support service?"

Vance has overseen several high-profile sex-assault investigations since he took office in 2010, including a recent case in which a female executive at the Rev. Al Sharpton’s National Action Network accused prominent civil rights attorney Sanford Rubenstein of rape.

Following a three-month probe, the DA’s office announced on Monday that it would not bring charges against Rubenstein, citing a lack of evidence.

In 2011 Vance also withdrew charges against ex-International Monetary Fund chief Dominique Strauss-Kahn after prosecutors found his accuser, a hotel maid, to be untruthful.