A veteran Oak Bluffs police sergeant has resigned following a tangled internal investigation into a police department-issued rifle that went missing for more than two years and was later found beneath piles of equipment in the basement of the town department.

Sgt. Michael Marchand will resign effective Feb. 2 with back pay for unused vacation time, according to a mutual separation agreement released by the town Thursday morning. A 16-year veteran of the department, Sergeant Marchand has worked in law enforcement on the Vineyard for 30 years.

A press release from the select board that accompanied the release of documents, including lengthy reports written by a Pembroke private investigator, said the circumstances surrounding the missing firearm will likely never be fully known.

“The missing rifle was . . . assigned to Sergeant Marchand and while we may never know the exact whereabouts of the weapon at all times over the past few years . . . the sergeant violated department rules when he failed to properly care for and account for the weapon,” the press release said in part.

The release of documents Thursday extensively chronicle the case of the missing so-called long gun, a Bushmaster model XM15-E2S M-4 patrol rifle.

Private investigator Paul L’Italien wrote a report in November 2021, was called back for further investigation after the gun was found and wrote a supplemental report in December 2021.

But in the end the exhaustive investigation led no clear conclusion about what had transpired.

According to Mr. L’Italien’s report, the rifle was issued to Sergeant Marchand in December 2016; at the time he was working as a school resource officer.

The report further recounts that Oak Bluffs Lieut. Timothy Williamson retired in October 2021 and Sergeant Marchand was assigned to take over some of the lieutenant’s duties, including tracking firearms. During a review of the firearms ledger, which was kept using a hand-written journal and Microsoft Word document, the sergeant discovered that records showed the gun was still in his possession, the report said. The sergeant notified police chief Erik Blake of the discrepancy, the report also said.

Soon after, Mr. L’Italien was hired to conduct an independent investigation, documents released Thursday show.

The investigator traveled to the Vineyard the following month. After interviews and a review of the records, he concluded in a written report that Sergeant Marchand had violated a series of police department protocols by failing to adequately protect the weapon from loss or misuse. In interviews with the private investigator, Sergeant Marchand cited poor recordkeeping and other lapses inside the department, but ultimately the report found him responsible.

“It is Sergeant Marchand who is responsible for the security of the weapons issued to him,” Mr. L’Italien concluded.

Dated Nov. 18, 2021, the report by Mr. L’Italien also recommended the department adopt more modern record-keeping methods for the use of weapons inside the police department.

On Dec. 14, 2021, the town issued a brief press release that the missing weapon had been found, and that an investigation remained ongoing.

Documents released Thursday show Mr. L’Italien was called back for further investigation.

In a Jan. 11 supplemental report made public Thursday, the private investigator gave a lengthy account of the work he conducted in December after the missing weapon had been found.

“The rifle was found under some large bags in a room which had been searched in October 2021 when the rifle was first reported missing,” the investigator wrote. “The rifle was discovered . . . inside a rifle case and the rifle was coated with rust, dirt and mold,” the report also said.

From there the continued investigation grew murky with numerous twists and turns. Among other things Mr. L’Italien interviewed numerous other officers inside the department, including one who had been assigned the rifle during training off-Island in the summer of 2019.

In the end the investigator said he could draw no clear conclusions.

“Unfortunately, the answers to the questions surrounding how the patrol rifle arrived in the lower-level training room of the police department, by whom it was placed there and at what time it was placed there are all undetermined,” Mr. L’Italien wrote.

“The investigation has established more questions than answers and unfortunately the new questions have not been answered,” he also wrote.

The investigator recommended that the case be closed.

The five-page mutual separation agreement between Sergeant Marchand and the town released Thursday includes non-disparagement clauses on both sides, stipulations that there will be no further public comment and other legal language. Sergeant Marchand will be paid $40,357 for 586 hours of unused vacation and compensatory time, according to the agreement. His letter of resignation was also attached.

The press release issued Thursday by the town further cited “a litany of false claims [that] appear to have been made by one of the sergeant’s lawyers in what may have been a misguided negotiation strategy aimed at securing a more favorable severance agreement. Quite frankly, this did not succeed.”