NEW YORK CITY — A small donation of $250 to the campaign of Manhattan District Attorney Cyrus Vance Jr. highlights nagging questions about how his office vets contributions for potential conflicts of interest. The donation came from a defense attorney on the day a consequential motion was filed in a disturbing sexual assault case, which ended with a plea deal.

Revelations that Vance's office declined to charge Hollywood mogul Harvey Weinstein after an NYPD sex assault investigation have ramped up scrutiny on donations to Vance, who won re-election by a landslide in 2013 and is running unopposed in next month's election. The Weinstein case surfaced days after three news outlets reported that Vance also received a donation in 2013 from an attorney associated with Donald Trump Jr. and Ivanka Trump, after his office closed an investigation into allegations the Trumps intentionally misled investors about the value of the Trump SoHo hotel. That donation was returned this month after questions were raised.

In 2015, Vance's office was prosecuting the case of an obstetrician accused of sexually abusing six patients.

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One woman told prosecutors that when she was 30 weeks pregnant Dr. Robert Hadden pulled her to him, and forcefully pulled her pants and underwear down.

"He then grabbed her buttocks, while rotating her body around, squeezing and grasping, 'cupping and manhandling' her buttocks, hips and vagina," according to court records. "He was not wearing gloves, and she felt his fingers in her vagina. Shortly thereafter, he excused himself for a few minutes, and then returned with his face red, flushed and sweaty. No nurse was in the room during this encounter."

She left and never returned, forced to find a new doctor in the third trimester of her pregnancy.

Court records indicate prosecutors working for Vance's office found 18 other women who said they suffered similar assaults during examinations by Hadden over the course of two decades. Five told prosecutors he would dismiss a nurse as they lay in the exam room, before putting a drape over their legs. His head would disappear and suddenly they would realize his tongue and fingers were probing them.

Robert Hadden Columbia University Medical Center

They came for routine pregnancy care and HPV exams. A dozen of the women reported what they believed were inappropriate breast exams. Two said they were asked to disrobe and bend over, in order to be examined from behind. Prosecutors said Hadden inappropriately touched the genitals of 13 of the women. One patient spoke only Spanish, and a nurse reported the alleged assault, according to court records.

Hadden was indicted in 2014 for six of the alleged assaults, but prosecutors intended to introduce evidence from the other 13. On Oct. 9, 2015, Hadden's attorney, Isabelle Kirshner, filed a lengthy motion opposing the prosecutor's plan. Later that day, she donated $250 to Vance's reelection campaign.

In a phone call with CBS News, Kirshner said the case's resolution had nothing to do with her donation or those of her firm. She said she doesn't recall the donation or its timing, but defended it nonetheless.

"This firm supported Cy because we believe he's the best person for the job, and we continue to believe he's the best person for the job," Kirshner said, adding that she's known Vance since 1982 and considers him "above reproach."

She also defended the practice of defense attorneys donating to the district attorney.

"The truth is that every criminal defense attorney, or at least many of us, have made donations. Frankly, nobody else really cares about that race," Kirshner said.

Although both Vance's campaign and office say they have long screened donations for appearance of impropriety, the donation from Kirshner was not flagged. Nor were donations totaling $3,000 contributed by Kirshner and the two namesake partners of her law firm Clayman and Rosenberg, just weeks after Hadden was sentenced, which the International Business Times first reported.

Vance's campaign says that over the years it has returned or declined $134,000 in donations from 31 would-be contributors, which the campaign said in a statement to CBS News was "well above and beyond" what it's required to do by law. District Attorneys in New York State are not barred from receiving donations from defense attorneys, even when they're arguing cases against those district attorneys. Individuals are allowed to donate as much as $50,000 in an election cycle to a district attorney.

Neither Vance's office nor his campaign explained why Kirshner's donation, which was made at the height of a sex abuse case, was not flagged, but Vance announced on Sunday that he is suspending all donations while a nonprofit anti-corruption center at Columbia University conducts an independent review of his operation.

The announcement came two days after Vance defended his office, saying money has never impacted decisions he's made as a prosecutor, while also pledging to "reexamine" how his office screens donations from attorneys.

"It obviously calls for an opportunity, like this for me, to rethink with my assistants how we wish to handle this matter going forward," Vance said at the John Jay College of Criminal Justice in New York City on Friday. "It's absolutely legal, but it doesn't mean that it shouldn't be reexamined office by office."

That Kirshner's 2015 donation wasn't returned highlights just how flawed the district attorney's system has been, said Sonia Ossorio, President of National Organization for Women, which held a protest outside Vance's office on Oct. 13.

"This is yet another alarm bell. It raises the question of whether the women of New York can have confidence that the Manhattan District Attorney is going to prosecute sexual misconduct to the fullest extent of the law," Ossorio said in an email to CBS News. "These are grave concerns that must be addressed."

Attorneys for Weinstein and the Trumps contributed tens of thousands of dollars to Vance before and after he made decisions related to their cases, revelations that have led state lawmakers to consider legislation limiting how much attorneys can contribute to district attorney campaigns. Earlier this month, after questions were raised, Vance returned the 2013 contribution he received from Trump's lawyer after closing the Trump Soho investigation. Vance told ProPublica the decision to drop the investigation had nothing to do with donations from the lawyer, and that he had determined that his office could not prove a crime was committed. Vance has similarly stated that donations had not influenced his decision in the Weinstein case, and that his sex crimes prosecutors had determined that the case was not provable.

Vance has taken in more than $6 million since he began his first run for district attorney in 2008. In the most recent election cycle, nearly half of the contributions to his campaign have come from attorneys or their law firms.

In reply to questions about the Hadden case, Joan Vollero, a spokesperson for Vance's office described the vetting process used by the office.

"Since 2010, the Office has designated a senior member of the DA's executive team with knowledge of the full universe of the office's investigations to review each contribution received by the campaign. When this individual determines that a contribution should be returned in order to avoid a potential conflict of interest, or the appearance of a conflict of interest, he or she informs the campaign to return it, without comment or explanation," Vollero said. "Had there been any conflict with any contribution, it would have been flagged and returned. This vetting process goes far beyond what is required by law, and no outside attorney, at any time, influenced any case-related decision."

While nearly all of New York State's 62 district attorneys accept donations from defense firms, contributions from attorneys who are arguing cases against those DAs raise particular ethical concerns, said James Gardner, a professor at the University at Buffalo who specializes in election law.

"It smells a little bit. There are two ways to handle ethics, one is to always charge right up to the line of what's acceptable, and the other is to keep a wide berth from that line. I suppose if you're running unopposed you're probably wiser to stay away from that line," Gardner said.

"There are people who say the DA shouldn't accept a donation from lawyers whose clients are caught up in the justice system," Gardner added.

In the Hadden case, Kirshner's filing on the day she donated to Vance's campaign was the final motion filed before a plea deal was announced.

Hadden never spent a day in prison.

He agreed to forfeit his medical license and enter guilty pleas to two of the counts, criminal sex act in the third degree and forcible touching. In exchange prosecutors dropped all other charges and agreed not to pursue cases related to any other alleged victims. The plea deal also included a stipulation downgrading his sex offender status to the lowest level — meaning he is not listed in New York State's online sex offender registry.

Kirshner's website highlights the case among her recent successes, noting the plea deal was "a favorable result for a doctor accused of sexually abusing multiple patients."

After the publication of this story, Vollero emailed the following statement to CBS News.

"Obtaining a felony conviction was the goal in this case. Had the case gone to trial, that may well not have happened. Typically, evidence gets weaker with time, not stronger," wrote Vollero. "The defendant pleaded guilty to felony and misdemeanor sex crimes. As conditions of his plea, he had to register as a sex offender, surrender his medical license, and waive his right to appeal in the future. He now has a felony conviction, which was not a guarantee had the case gone to trial."

Volero also said Kirshner never spoke with Vance about the case, but did meet with Chief Assistant Karen Friedman-Agnifilo, a former Sex Crimes Unit Deputy Chief, who said prosecutors were ready to go to trial unless Hadden agreed to plea guilty to a felony. Volero noted that prosecutors wanted a felony conviction and that it would have been at risk if the case went to trial. She also said statutes of limitations may have led the judge to dismiss some charges in the indictment. The minimum sentence for a class E felony in New York is nothing, she noted, while the maximum is between 16 months and four years.

In a statement sent about an hour after Vollero's email, Friedman-Agnifilo said she, not Vance, made the final decision.

"Despite the defense attorney vigorously advocating for her client and pointing out significant proof issues with the case, we ultimately refused to offer the misdemeanor plea she was seeking," Friedman-Agnifilo said. "We insisted that the doctor plead guilty to a felony and give up his medical license so he could no longer abuse women in his care. The decision was made by me, in consultation with the prosecutors on the case, and none of us have or had any idea about any campaign contribution by anyone including this attorney."

Got news tips about criminal justice or campaign finance? Email this reporter at KatesG@cbsnews.com, or via his encrypted address, grahamkates@protonmail.com (PGP fingerprint: 4b97 34aa d2c0 a35d a498 3cea 6279 22f8 eee8 4e24).