A Vineyard prosecutor and Cape and Islands assistant district attorney will serve a one-month suspension for misconduct following a decision this week by a single justice of the state Supreme Judicial Court.

The Monday ruling by the Hon. Elspeth B. Cypher marks the end of a complicated and unusual investigation by the state Board of Bar Overseers involving Laura Marshard, a West Tisbury resident and longtime assistant district attorney. Ms. Marshard has been under public scrutiny since 2016 for alleged misconduct in three criminal cases she handled in 2013 and 2014.

A petition for discipline was filed against her with the BBO in September 2016. The legal proceeding that followed before the BBO, a state board that investigates complaints against attorneys, went on for more than a year. It included a hearing that took place over eight days in May and June of 2017, testimony from 14 witnesses, hundreds of pages of transcripts and 76 exhibits. Two Vineyard defense attorneys who are brothers — Timothy and Robert Moriarty — were among the witnesses who testified. Robert Moriarty represented a defendant and Tim Moriarty represented a witness in one of the cases that was the subject of the BBO investigation.

Two of three counts in the case were eventually dropped. In the end the BBO found that Ms. Marshard had violated the rules when she met with a witness in a 2014 case involving assault charges without having his attorney present.

The three-member panel that heard the case recommended a reprimand, but the full board later recommended a one-month suspension, citing among other things Ms. Marshard’s long experience as a prosecutor.

Under the process, the final decision rested with a single justice of the SJC.

In a memorandum of decision issued Monday, Judge Cypher upheld the one-month suspension and also said Ms. Marshard had satisfied a requirement that she attend continuing education by completing two courses on ethics in the law.

“Having considered the underlying facts, the respondent’s acceptance of responsibility and acceptance of sanction, as well as considering discipline imposed in other cases, I conclude that the one-month suspension was appropriate and the that the courses attend by the respondent satisfy the CLE (continuing legal education) requirement imposed by the board,” the judge wrote.

Ms. Marshard’s attorney Elizabeth Mulvey said her client is relieved to have the proceeding come to an end. “It’s been, obviously, a horrendous experience for her. She appreciates the support she’s received from her office, and she’s looking forward to putting it behind her,” Ms. Mulvey said.

The attorney for the BBO who argued the case was unavailable for comment.

In a lengthy statement, Cape and Islands district attorney Michael O’Keefe said Ms. Marshard would not appeal the ruling. “This is a personal decision made with concern for her family who have endured these proceedings, along with ADA Marshard, for years,” Mr. O’Keefe wrote. But he also defended his assistant attorney in passionate terms: “While attorney Marshard accepts the board’s recommendation and I understand her position, the findings of fact by this tribunal are not supported by the evidence elicited at the proceeding.”

Mr. O’Keefe singled out the Moriarty brothers for especially harsh criticism. “Any experienced practitioner knows what was happening here,” the D.A. wrote, harking back to the three criminal cases that were at the center of the original complaint against Ms. Marshard. “These two brothers, lawyers with very little regard for the rules, were trying to gain an advantage by keeping the victim from testifying, thereby enhancing the defendant’s chances that he would not be prosecuted. It is an unhealthy situation when two brothers, both of whom are the sons of a superior court judge sitting on the Island . . . all practice on the tiny Island of Martha’s Vineyard. The rules of professional conduct contemplate many of the scenarios occasioned by familial relationships among lawyers but they have to be followed in order to work. The people of Martha’s Vineyard deserve no less.”

In a statement emailed to the Gazette later, Robert Moriarty responded. “I am saddened and disappointed by the unhinged personal attacks levied by District Attorney O’Keefe that were clearly meant to obfuscate and distract from the findings of the board, ” he wrote. “It does not befit an elected district attorney to falsely smear lawyers summonsed to testify before the Board of Bar Overseers in an investigation into prosecutorial misconduct simply because we had the temerity to tell the truth.”

Judge Cypher ordered that the suspension begin one month from Oct. 22. The order says Ms. Marshard cannot accept any new cases during the month before the suspension begins, and must wrap up all legal work or notify clients that they need to get another attorney.

Ms. Marshard was admitted to the bar in 1991. She will be eligible to have her license to practice law reinstated after serving the one-month suspension.