The federal government refused to grant immunity to Nassau County Executive Edward Mangano, who has threatened to take the Fifth Amendment if called to testify at the political corruption trial of former state Senate leader Dean Skelos, according to a transcript of proceedings Friday.

Kenneth Sherrill, a professor emeritus of political science at Hunter College who’s been following the trial, said an elected official’s threat to invoke his Fifth Amendment rights “is suicidal” and makes him “seem radioactive.”

“It’s interpreted as an admission of guilt,” Sherrill said. “It’s pretty easy for other politicians and for journalists to connect the dots as to why he would be taking the Fifth.”

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Mangano lawyer Kevin Keating of Garden City declined on Saturday to answer questions but said in a prepared statement: “Mr. Mangano is merely a fact witness in the Skelos matter and if called to testify he will provide truthful testimony, even though Mr. Mangano has engaged in no wrongdoing. If he is asked questions about unrelated matters, I have counseled him to invoke his Fifth Amendment privilege,” a position Keating said is “fully appropriate” under the law.

Mangano spokesman Brian Nevin said in a statement Saturday that his boss “was never on the government’s witness list and never sought immunity.”

Dean Skelos attorney Robert Gage made the disclosure about Mangano at a sidebar on Friday in court, during testimony by Mangano’s chief deputy, Rob Walker, who did receive immunity. Neither the judge nor the prosecution disputed Gage’s statement.

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Mangano and Walker have both been identified in Newsday stories as being under scrutiny in an unrelated corruption probe by federal prosecutors on Long Island stemming from charges against restaurateur Harendra Singh, who was a major campaign contributor to Mangano and others on the Island.

Both Mangano and Walker were viewed as potential witnesses in the Skelos case, brought by different federal prosecutors in Manhattan, because court filings indicate that both had contacts with alleged efforts by Skelos and his son, Adam Skelos, to push a $12 million storm water-pollution contract with Nassau County for AbTech Industries, a firm that hired the son.

Walker, who testified Friday, said he would assert his Fifth Amendment right and testified under an immunity order signed by U.S. District Judge Kimba Wood, which assured him that his testimony in the Skelos trial would not be used to prosecute him for anything unless he committed perjury, but gave him no assurance that he won’t be prosecuted in the Long Island investigation based on other evidence.

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Mangano, according to the sidebar statement by Skelos lawyer Gage, took the same position as Walker — threatening to take the Fifth — and the government decided to not grant him immunity.

Jim Margolin, a spokesman for U.S. Attorney Preet Bharara, whose Manhattan office is prosecuting the Skeloses, declined to comment on why the government was willing to grant immunity to Walker, but not Mangano.

Brian Griffin, Walker’s lawyer, also declined to comment on the different decisions the government made about the two Nassau officials. He did say the immunity was justified for Walker, and was a prudent legal step that did not mean he was truly at risk of being charged.

“Although Mr. Walker has done nothing wrong, securing immunity was without question the only appropriate legal procedure to take,” Griffin said. “Becoming a public official does not mean the loss of your constitutional rights.”

Although Walker is technically still exposed to charges, Gage in his questioning Friday suggested his risks are in practical terms now over. Legally, after testimonial immunity is granted, the process of prosecuting someone based on unrelated evidence becomes more difficult for the government.