A Martha’s Vineyard-based prosecutor with the Cape and Islands district attorney’s office is facing possible disciplinary action by the state agency that investigates complaints against lawyers.

A complaint filed by the Office of the Bar Counsel alleges that assistant district attorney Laura Marshard committed professional misconduct in her handling of three separate criminal cases on the Vineyard, including violating rules that require disclosure of evidence that could help the defense.

The bar counsel, which investigates and prosecutes complaints against lawyers, has requested a hearing before the state Board of Bar Overseers on the matter. In a detailed response filed last week by her attorney, Ms. Marshard disputed each of the charges and called for the case to be dismissed.

Filed in September, the complaint describes three alleged instances, or counts, of misconduct. The first stems from a 2014 jury trial prosecuted by Ms. Marshard in Dukes County superior court that involved an alleged assault during a brawl at the Ritz in March 2013. Defendants Patrece Petersen and Darryl Baptiste faced assault charges, and Mr. Petersen also was charged with attempted murder.

In an unusual step during the trial, the Hon. Richard Chin halted proceedings to admonish Ms. Marshard for misconduct. Attorneys for the defendants claimed at the time that key witnesses were concerned about testifying because they might be prosecuted, and that certain actions by Ms. Marshard may have been prejudicial.

Judge Chin said from the bench that Ms. Marshard improperly hindered the defense but he did not dismiss the case. Both men were later acquitted by the jury.

The bar counsel complaint focuses on this and other parts of that trial, including an allegation that Ms. Marshard did not disclose information she obtained in an interview with the bartender at the Ritz.

The complaint further alleges that Ms. Marshard did not take steps to interview other witnesses in the case, and that she told the court that defense witnesses might have concerns about self incrimination if they testified. The complaint claims Ms. Marshard’s statements were without a good faith basis and attempted to interfere with the defendants’ ability to present their case to the jury.

A second count centers on a case involving an alleged fight in July 2014 between Mr. Petersen and another man, Dave Sylvia. Mr. Petersen faced several charges and a probable cause hearing was scheduled in the case, according to the complaint. Ms. Marshard allegedly met with Mr. Sylvia prior to the hearing to discuss his fifth amendment rights against self-incrimination and to assure him he wasn’t in trouble; according to the complaint she failed to seek permission from his counsel to do so.

The third count centers on an August 2013 drug investigation in Oak Bluffs in which Nicholas Viaggio was arrested on drug and larceny charges. According to the complaint, a drug task force detective used Mr. Viaggio’s cell phone to text another suspect who had been approached by police. After the text was sent, the detective observed the other suspect’s phone make an alert-type sound, the filing states.

According to the complaint, Ms. Marshard later called state police Sgt. Jeff Stone to testify before the grand jury about the investigation, and asked him to recount parts of the detective’s report. The complaint alleges that Sergeant Stone testified that the text sent to the suspect was visible on the phone. The complaint states that Ms. Marshard “knew that Stone’s testimony was false and failed to take the reasonable remedial measures to correct it.” Or if she was not aware the statement was false, she was not adequately prepared to present the matter, the complaint said.

Represented by her attorney, Ms. Marshard refuted all three counts in detail in a 16-page document filed in the case last week. Among other things, she said that because the bartender was not an eyewitness to events, failure to disclose that conversation did not unfairly harm the two defendants. Moreover, she said, she legitimately believed when asked that the two potential defense witnesses might have had concerns about self-incrimination.

The response also said Ms. Marshard spoke to Mr. Sylvia for the sole purpose of explaining that the judge had appointed counsel for him. The response noted that the Hon. H. Gregory Williams conducted a hearing on the matter in September 2014, and did not find improprieties or impose sanctions. She also stated that Sergeant Stone’s testimony was not knowingly false, nor did she know that it was false, and that she had fully prepared for his testimony.

In an emailed statement to the Gazette Monday, Cape and Islands district attorney Michael O’Keefe said he “fully supports’’ Ms. Marshard, who has worked as an assistant district attorney for 12 years and has had no previous issues with the Board of Bar Overseers. “Ms. Marshard maintains she committed no breach of the canons of ethics and looks forward to an opportunity to establish that,’’ the statement said.

Speaking briefly to the Gazette by phone Monday, Constance V. Vecchione, chief bar counsel, confirmed that the proceeding is somewhat out of the ordinary.  “Prosecutions of prosecutors are not common,’’ she said.

Ms. Marshard is a resident of West Tisbury.

A spokesman for the Board of Bar Overseers said the agency is in the process of scheduling hearings in the matter.