Suffolk police are investigating a complaint filed by a retired Nassau police sergeant who said two Suffolk detectives went to his home last year and asked him to “help out” Oheka Castle owner Gary Melius.

The retired Nassau sergeant, Sal Mistretta, said that, perplexed by the visit, he called one of the detectives and got into a heated argument, leading him to lodge a formal complaint with the Suffolk County Police Department alleging that he had been threatened during the call.

During the visit, the detectives never specified what help they were seeking because Mistretta said he wanted to speak to his lawyer and shut the door on them, Mistretta said.

Suffolk police spokesman Justin Meyers said he could not comment or provide records related to the visit, requested by Newsday, because of the internal investigation, but he said it was opened in June 2015 immediately after a civilian filed a complaint. Asked why it had not been concluded after 15 months, he said, “We are very, very comprehensive in our investigations.”

What the detectives wanted Mistretta to do remains a mystery, the latest involving Melius, a Long Island political power broker who was shot in the head in February 2014 while leaving Oheka to meet former Sen. Alfonse D’Amato for lunch. The shooting remains unsolved.

Melius’ attorney, Joseph Tacopina, said Melius knew nothing about the incident involving the detectives and had no comment.

The two detectives work in the Second Precinct in Huntington, about 5 miles from Oheka, Melius’ headquarters, where he has hosted dinners for political luminaries and police. He has long had cordial relations with police on Long Island and around the state.

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The internal investigation is being conducted by an officer in the Second Precinct, which Meyers said was standard procedure. An internal investigation considers minor complaints. Meyers said it was appropriate because the civilian had complained that “an officer spoke inappropriately to him.”

Meyers was referring to a phone conversation Mistretta had with one of the detectives after the visit to his home. Mistretta taped the call and provided the audiotape to Suffolk police and more recently to Newsday. Mistretta said he went public because he was frustrated that it is unresolved.

Jeffrey Noble, a police consultant who has written extensively about police accountability, said an investigation into a taped phone call should take two hours, raising a question about why it has taken more than 15 months.

Meyers declined to comment on whether the department was concerned about the allegation that detectives appeared to act on Melius’ behalf.

Suffolk’s investigation began after Det. Michael Volpicelli and another detective arrived unannounced on Mistretta’s doorstep about 10 a.m. on June 15, 2015.

Mistretta gave this account: They blocked his driveway with their car and stood with holstered guns and badges displayed in a way that frightened him. He said Volpicelli asked him, “Can you help out Gary Melius?”

Startled, Mistretta said, “What?”

Volpicelli repeated his question, he said. Worried they might be impostors, he asked for Volpicelli’s card, which he gave him. He shut his door, and they left.

That same day, Mistretta said he called Volpicelli and asked him why they wanted him to assist a civilian. He said Volpicelli started yelling. Mistretta hung up.

Alarmed, he called his attorney, who advised him to call Volpicelli again and tape the conversation. On the call, Mistretta asked to speak to Volpicelli’s supervisor. The exchange became heated, with Mistretta telling Volpicelli to “talk like a man.”

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In response, Volpicelli shouted, “Don’t tell me to talk like a man, you [expletive] little [expletive]. I’ll come there right [expletive] now and we’ll talk, how about that? How about that?”

Mistretta thanked him, told him the call had been recorded and hung up.

He said he then called Volpicelli’s supervisor, Det. Sgt. John O’Sullivan, who he said berated him and referred to Newsday stories that had been critical of Mistretta.

O’Sullivan and Volpicelli declined to comment through a department spokesman.

In 2013, Mistretta was enmeshed in a political-influence scandal that involved Melius and cost Nassau Police Commissioner Thomas Dale his job.

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While off duty, he served a subpoena on Randy White, a campaign worker for former Freeport Mayor Andrew Hardwick, who received more than $23,000 in campaign contributions from Melius. White had testified during a court challenge that Hardwick’s campaign had violated election law.

An investigation by then-District Attorney Kathleen Rice found that Melius told Dale the campaign wanted perjury charges filed against White.

Police could find no evidence of perjury. But on Dale’s orders, they arrested White on an unrelated misdemeanor.

Rice’s investigation found police committed no crimes by arresting White, but it questioned Dale’s judgment. He resigned shortly after, and Mistretta retired in November 2013.

Mistretta later said that he served the subpoena merely as a favor to a friend and was an unwitting participant in the scandal.