In a rare move, the judicial office that brings disciplinary cases against lawyers in Massachusetts has accused a prosecutor of professional misconduct, including allegations that she failed to share critical information with defense lawyers and attempted to interfere with defense witnesses.

Laura Marshard, an assistant district attorney on Martha’s Vineyard, faces allegations related to behavior in three cases, including an ongoing trial. If sanctioned, Marshard would be the first Massachusetts prosecutor disciplined for trial conduct since 2009. The allegations, filed in September by the Office of the Bar Counsel, offer a rare glimpse into the state’s disciplinary process for prosecutors, who are seldom accused or investigated.

An Eye review of board data found just 11 state and local prosecutors have been disciplined for trial conduct in the past four decades. The Board of Bar Overseers, which adjudicates bar counsel complaints, has disciplined more than 1,300 attorneys overall since 2005. None of the 11 prosecutors who faced discipline knowingly withheld evidence, as Marshard is charged with doing. Marshard denied the bar counsel’s allegations in her formal response filed last.

She declined to comment on the disciplinary case, according to an e-mail from Tara L. Miltimore, spokeswoman for the Cape and Islands District Attorney’s Office. Marshard has worked in that office for 12 years. In her filing, Marshard called her career “exemplary” and said she had never been sanctioned. She previously worked for six years as a Suffolk County prosecutor and for seven years in Colorado.

R. Michael Cassidy, a former state assistant attorney general who teaches legal ethics at Boston College Law School, said bar disciplinary committees nationwide “are beginning to commence proceedings against prosecutors more frequently,” particularly regarding failure to disclose evidence — and Massachusetts may be joining the trend.

Cassidy, a former hearing officer with the Board of Bar Overseers, has criticized the body for a “historic trend” of deference to prosecutors. Cassidy said the petition against Marshard may signal a reversal of such deference. The lead allegations against Marshard stem from a bar brawl in Oak Bluffs in March 2013 that led to assault indictments for two defendants, identified in court records as Patrece Petersen and Darryl Baptiste.

Petersen was also indicted on charges of assault with intent to murder. Both defendants pleaded not guilty, and in October 2014, their attorneys moved to dismiss all charges after a bartender disclosed that the prosecutor interviewed her about the incident. Marshard didn’t tell defense attorneys about the bartender’s account, according to the Office of the Bar Counsel.

In an affidavit, the bartender, Christine Arenburg, said she told Marshard that the alleged victims, Frank and Jason Cray, “lied about the whole thing.” The defendants’ lawyers argued that Marshard’s failure to share the bartender’s account was “egregious prosecutorial misconduct.” The prosecution’s obligation to share potentially exculpatory information was enshrined in the US Supreme Court’s landmark 1963 ruling in Brady v. Maryland.

In her response motion at the time, Marshard argued that much of the bartender’s brief statements were little more than guesswork because she had not seen the fight. In her filing in the bar proceeding, Marshard said the bartender’s statements weren’t valid evidence and weren’t of use to defendants. Dukes County Superior Court Judge Richard Chin ruled from the bench in October 2014 that Marshard’s “late disclosure” of her meeting with the bartender constituted prosecutorial misconduct, and the bartender’s statement “certainly was something the defense should have been made aware of.”

The judge found Marshard further prejudiced the trial by asking the court to advise defense witnesses of their Fifth Amendment right against self-incrimination. The judge said the request was a strategic move designed “to prevent the defendants from presenting any kind of defense.” Marshard told Chin she was not seeking to “harass or intimidate” the witnesses. She said in her bar response that she was obligated as a prosecutor to raise concerns over potential self-incrimination.

The judge did not dismiss the case, and a jury acquitted both defendants in November 2014. Marshard also faces misconduct allegations in two other recent cases. In an assault trial that is ongoing — in which Petersen is also the defendant — the bar counsel said Marshard met with an alleged victim without alerting Petersen’s court-appointed attorney.

Such a meeting runs afoul of two rules of professional conduct, according to the disciplinary petition. Marshard was seeking to assure the alleged victim “that he was not a target of the district attorney’s office,” according to her recent filing. The third case stems from a 2014 drug-trafficking indictment of three men arrested in Oak Bluffs with nearly 300 oxycodone and Percocet painkiller pills worth more than $14,000.

The matter never went before a jury: One defendant was murdered, and charges against the other two were dropped. The bar counsel alleges Marshard failed to correct false testimony by a state police sergeant, who was a witness before the grand jury that returned the indictments. The defendants, in a motion to dismiss, flagged an inconsistency between the sergeant’s grand jury testimony and the arresting officer’s testimony during a hearing. T

he state police sergeant told the grand jury that the arresting officer saw a text message sent from a cooperating drug dealer’s phone appear on one of the defendants’ phones. But the arresting officer testified he only observed that the phone buzzed, and had not actually seen the text message. The defendants accused Marshard of negligently failing to correct the record and said that may have swayed the grand jury to indict.

At the time, Marshard argued that the sergeant did not knowingly present false evidence and that the grand jury had plenty of evidence to issue an indictment. The district attorney’s office dropped the charges, telling the court it was unable to sustain its evidentiary burden. The bar counsel contends that Marshard either knew the sergeant’s testimony was false and “failed to take reasonable remedial measures to correct it,” or else missed the conflicting accounts because she failed to prepare adequately for grand jury.

Either way, the petition claims she violated numerous professional conduct rules. Her bar filing denied that she knew the testimony was incorrect or that she was ill-prepared. The next steps in the case will be hearings before the Board of Bar Overseers.

Intern Clairissa Baker contributed to this report.