Tisbury selectmen have ordered a comprehensive report into operation of the town police force, driven by internal conflicts which this week forced Chief John Cashin from his job.

Mr. Cashin stepped down from the position on Wednesday, less than a week after he labeled his department dysfunctional and accused selectmen of undermining his authority with the force.

His departure, engineered at two extraordinary meetings of the selectmen on Friday and Monday, came two and a half months before the expiration of his three-year contract.

The town now begins a search for a new chief — the third in three years and fourth in seven years for the troubled department.

After yet another board meeting on Tuesday night, it was announced on Wednesday that patrolman Dan Hanavan would act as interim chief, beginning at 5 p.m. that day.

And then early Thursday, the selectmen announced a decision to initiate a “structural assessment” of the police department, to be conducted by Robert Wasserman, who also was retained in 2001 to report on the operations of the Tisbury department.

A resident of West Tisbury, Mr. Wasserman has held numerous police executive positions and served as a consultant to several police forces here and in other countries, most recently in London.

The circumstances which have brought the selectmen to Mr. Wasserman again are strikingly similar to those which were the subject of his investigation in 2001.

That report found the department to be “dysfunctional at best.”

It enumerated problems including long-term resentment of leadership by members of the force, disgruntled officers going behind the back of the chief to selectmen, a force split into two hostile camps and a campaign by some to have the chief dismissed.

In 2001, as now, the situation was complicated by complaints from officers to the Massachusetts Commission Against Discrimination. And one significant point of tension then, as now, was an attempt by the chief to discipline an officer for abuse of sick leave entitlements.

The 2001 Wasserman report found some officers had engaged in behavior which should have merited discipline or discharge but which had gone unpunished “probably because [they] figured out that threatening litigation (or charging intimidation) can focus attention away from their misbehavior.”

The report recommended a number of measures to improve matters, all of which were subsequently adopted according to the chairman of the selectmen, Tristan Israel, who also was a selectman at the time.

The report concluded: “Simply firing the chief of police . . . will not help.”

In his interview with the Gazette last Thursday, Chief Cashin also described the department as dysfunctional.

He said a handful of police had been hostile toward him ever since his appointment, and had spread “vile, preposterous and outrageous” rumors about him.

About five members of the 13-strong force were involved in a whisper campaign against him, he claimed, adding that he was contemplating legal action against some of them for defamation.

Chief Cashin said a tense relationship existed between the dissident group and the rest of the force and that he spent much of his time settling disputes between personnel.

He also accused one of the selectmen — whom he declined to name but who was identified by several other sources as Jeff Kristal — of undermining his authority by interceding on behalf of an officer Chief Cashin wanted to discipline for abuse of sick leave entitlements.

He challenged the selectmen to choose between him and his opponents within the department.

“I think the time has come to deal with the personnel issues that have been plaguing this department for years,” he said, adding:

“I heard a month after I got here this one particular sergeant was saying ‘I’m not concerned about the chief, the chief’s office has a revolving door. He’ll be out of here in three years.’

“I would like the support of the selectmen. And if they will have me, I will stay, and that will send a message to these officers that they are not empowered to evict a chief.”

Despite Mr. Cashin’s clear expression of a desire to stay in his position, selectmen insisted this week he had not been sacked. His early termination, the Tuesday statement said, was decided by mutual agreement. The terms of the agreement have not been made public.

In the statement, the selectmen expressed disappointment that Chief Cashin’s relationship with the town was ending.

It quoted Mr. Israel: “I speak for the entire board when I say that we have nothing but the utmost respect for Mr. Cashin and the improvements he’s made within the Tisbury police department.

“His refreshing approach to community policing, and dedication to law enforcement has been greatly appreciated.

“We thank him for his hard work and wish him nothing but the best in all his future endeavors.”

The following day’s statement, announcing the appointment of patrolman Hanavan as acting chief, said he would continue in that role until Oct. 12.

“The board will utilize this interim period to decide on a hiring process for the new police chief,” it said.

Mr. Hanavan holds a bachelor of arts degree from Syracuse University and a master’s degree in criminal justice from Northeastern University. He has lived in Vineyard Haven and been a member of the department since 1988.

Officer Hanavan was a finalist for police chief in 2006 when Mr. Cashin was hired. In choosing patrolman Hanavan, the selectmen passed over all three of the department’s sergeants, including the man who acted as chief before Mr. Cashin’s appointment, Sgt. Tim Stobie.

Sergeant Stobie is currently the subject of a sexual harassment and discrimination complaint lodged on April 28 with the Massachusetts Commission Against Discrimination (MCAD) by the force’s only female officer, Kelly R. Kershaw.

Officer Kershaw’s complaint remains under investigation by MCAD.

Meanwhile, selectmen continue to try to handle their fractious force as delicately as possible.

The announcement of the engagement of Mr. Wasserman stressed that they recognized the importance of concerns held by department members.

The selectmen had planned to deliver the news of the Wasserman review at a meeting with police yesterday, before going public with it.

But the meeting was cancelled because personal issues made it impossible for the acting chief, Mr. Hanavan, to attend. It has been rescheduled for Tuesday.

The 2001 Wassermen report said that the causes of the Tisbury police department’s problems could be traced back many years. This week, sources with intimate knowledge of the force over the intervening years said some things had changed. For example, with the sole exception of Mr. Kristal’s recent intervention, the selectmen had not interfered in personnel management.

But they said the root personnel issues continued to fester, despite the reforms and the efforts of three police chiefs and the selectmen over the intervening years.

Mr. Wasserman will begin work this week, starting with interviews of police and town officials.