The Nassau district attorney’s office has determined jail officials and the county executive’s staff mishandled an internal investigation involving a jail nurse accused of smuggling contraband into the facility, but the failures didn’t rise to the level of criminal behavior.

The revelation came from District Attorney Madeline Singas’ office in letters sent Monday to the leader of Nassau’s correction officer union after he asked for an investigation into whether Sheriff Michael Sposato or other county administration officials obstructed justice by allegedly not initially pursuing criminal charges against the nurse.

The former Armor Correctional Health Services nurse has pleaded not guilty since her arrest last February along with two other women, since convicted in the scheme.

Union leader Brian Sullivan’s appeal Friday for a probe, which he repeated at a Monday news conference, followed a Newsday story from January. The story reported that law enforcement sources said Sposato’s administration at first didn’t seek to arrest the Armor nurse and tried to handle the matter quietly in-house by having her fired after evidence emerged showing her involvement in a ring that smuggled razors and synthetic marijuana to inmates.

The arrests only happened after Singas’ office found out independently about the smuggling scheme, demanded a briefing from Sposato, and launched a probe that then included cooperation from the Sheriff’s Office, Newsday also reported.

Sposato repeatedly denied that version of events, and said Friday in a statement that he welcomed a district attorney’s probe into the “baseless accusations” in order “to put this issue to rest once and for all.”

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A spokesman for County Executive Edward Mangano released a statement Monday night from Charles Ribando, the deputy county executive for public safety. It said “the correctional center followed all proper procedures,” after an extensive internal investigation was done at the jail to try to identify the sources of possible jail contraband.

“An employee of Armor was confronted weeks later while entering the jail with contraband. Subsequently, she was questioned by internal investigators and shortly after the matter was turned over to the district attorney’s office with evidence,” he added.

Monday afternoon, the sheriff released a statement calling Sullivan a “weak and ineffective” union leader.

“The truth matters. What you say matters. How you lead matters,” Sposato said in the statement. “I stand on my record as the Sheriff.”

Documents Singas’ office released Monday showed jail officials and Mangano’s staff conducted an investigation spanning several weeks into the potential contraband smuggling in the East Meadow facility without involving police or prosecutors.

In a letter addressed Monday to Sullivan, Chief Assistant District Attorney Albert Teichman said prosecutors learned of the smuggling scheme in December 2015 when “multiple unofficial sources” alerted Singas’ office that an Armor employee “had been questioned regarding allegations that she smuggled contraband” into the jail.

Teichman wrote that Singas’ office immediately began an investigation that later resulted in the arrest of the nurse and two alleged co-conspirators.

“Troubled that the DA’s Office had not been properly notified, we reviewed the investigation that had been conducted internally, and found that while the matter was mishandled, the failures were noncriminal,” he added.

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Teichman also said that at the time, he sent County Attorney Carnell Foskey a letter formally requesting all county agencies be reminded the police or prosecutors “must be notified of any potentially criminal investigation.”

In that Dec. 30, 2015 letter — also sent Monday to Sullivan — Teichman wrote: “This is clearly a criminal matter and this office or the police department should have been consulted about this investigation at its inception.”

The December 2015 letter to Foskey also shows there was an allegation that when jail and county executive’s staff met with an Armor employee on Dec. 23, 2015 — a reference to their confrontation with the nurse — she “may have been told that she would not be prosecuted for certain criminal offenses in exchange for information.”

The letter went on to say that such an offer was beyond their authority. It also said the district attorney’s staff met with Sposato immediately upon learning of the matter on Dec. 28, 2015, the same day prosecutors also discussed the issue with Ribando.

Teichman’s letter to Foskey ended by saying all county agencies should be advised “they are required to report the investigation of potentially criminal conduct to the police department or this office to enable successful criminal prosecutions where warranted.”

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The contraband arrests came at a time when Armor, which has defended its care, was coming under fire in connection with a series of Nassau inmate deaths.

“To me, this vindicates the allegations,” Sullivan said Monday after getting both letters from Singas’ office. “They showed this matter was mishandled . . . Why would anybody from the county executive’s office be involved in questioning someone at the jail regarding a criminal matter for any purpose other than to insulate the sheriff from any more bad press regarding Armor?”