Nassau County Executive Edward Mangano and his chief deputy, Rob Walker, both plan to refuse to answer questions from federal prosecutors about their relationships with indicted Long Island restaurateur Harendra Singh, according to several sources.

The refusal, on the advice of their attorneys, before the Manhattan trial of Republican state Sen. Dean Skelos and his son, Adam Skelos, would make it unlikely that either official can be used as a government witness because of a Supreme Court ruling, the sources said.

Prosecutors typically do not want a witness who has declined to answer all questions because that could potentially undermine his or her credibility, the sources say.

Both Mangano and Walker, a deputy county executive, have figured prominently in the government's legal papers that led to the Skeloses' arrests and indictments.

The county executive, who other sources said had previously testified before the grand jury investigating the Skeloses, and Walker have every legal right to refuse to answer questions about Singh or Oyster Bay politics, the sources said.

Mangano is a longtime friend of Singh, who was indicted separately in September on a number of felony charges in the federal Eastern District of New York, including bribing a deputy Oyster Bay town attorney to obtain what amounted to millions of dollars in business loan guarantees from the town.

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Neither Mangano nor Walker, who also both have deep roots in Oyster Bay politics -- where the ongoing Singh federal probe has been investigating possible political corruption -- has been accused of any wrongdoing in either the Skelos or the Singh cases.

But the two otherwise separate cases are linked by a Supreme Court ruling in criminal cases, involving potential prosecution witnesses, the sources said.

Credibility questions

The ruling requires prosecutors to actively seek and turn over to defense attorneys any information -- known as Giglio material, after a Supreme Court case by that name -- that might be used to attack the credibility of prosecution witnesses.

Federal prosecutors are now reviewing potential witnesses in preparation for the Skelos trial, scheduled to start Nov. 16 in federal court in Manhattan.

Mangano's grand jury testimony in the Skelos case occurred before the indictment of Singh on Long Island, so prosecutors in the Southern District would have had no reason then to ask him during the Skelos investigation about Singh or possible questionable activities in Oyster Bay.

But in preparation for the Skelos trial, the prosecutors now would have an obligation to do so because of the Giglio rule, the sources said.

Federal prosecutors on Long Island have continued to look into other areas growing out of the Singh case, according to sources. According to emails, invoices and other documents obtained by Newsday, Singh has helped pay for Mangano's travels, and hired Mangano's wife, Linda, to assist in his restaurant business at a salary of $104,000 a year. He also comped Mangano frequently for meals at his restaurants, according to some of Singh's employees.

Nassau Deputy County Executive Ed Ward has said Edward Mangano always paid for his meals, and he has not been charged with wrongdoing.

At the stage of pretrial preparation for the Skelos case, neither Mangano nor Walker would have to invoke their Fifth Amendment privilege against self-incrimination to avoid answering questions about relationships with Singh or Oyster Bay, the sources said.

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Mangano and Walker could merely say they won't answer questions about Singh or Oyster Bay, or could refer prosecutors to their attorneys, even if they have been fully cooperative before, without legal jeopardy, the sources said.

It is only if they were actually put on the witness stand that they would have to invoke the Fifth to avoid the questions.

The sources said it would be imprudent for their attorneys to allow them to answer open-ended questions about Singh, the town or other acquaintances or associates in the face of an ongoing federal corruption investigation on Long Island, without knowing where it is leading.

Mangano's attorney, Kevin Keating of Garden City, and Walker's attorney, Brian Griffin of Garden City, both said their clients had not engaged in wrongdoing nor have they been accused of having done so.

Keating and Griffin, however, declined to comment on whether their clients are now cooperating with prosecutors in their Skelos pretrial preparation, and they repeatedly declined to answer whether their clients would refuse to discuss with prosecutors either Singh or Oyster Bay politics.

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Keating said in a statement, "Mr. Mangano, of course, will fully comply with any lawful obligation to provide truthful testimony at any proceeding. With regard to the Singh matter . . . Mr. Mangano has engaged in no wrongdoing."

Griffin said in a statement, "I am representing Mr. Walker in his capacity as a witness in the Southern District trial of Dean Skelos. To be clear Mr. Walker is neither a target or subject, but merely a fact witness. As has always been the case, Mr. Walker will give truthful and accurate witness testimony if called upon to do so."

Singh's former attorney, Joseph Conway of Mineola, has said that his former client has no information linking him to any political corruption.

Singh's current attorney, Anthony La Pinta of Hauppauge, said, "Mr. Singh is not involved in the Skelos case in any way. Any attempt to link him to that case would be inappropriate."

Careers linked

The careers of both Mangano and Walker are intertwined in Oyster Bay politics and government.

Mangano was a Nassau County legislator from Bethpage from 1996 until 2009, when he was elected county executive.

Oyster Bay Town Supervisor John Venditto has said that in 2011 he hired Mangano's brother Rob as deputy public works administrator on the county executive's recommendation, but added that the brother was highly qualified for the position, according to a Newsday article.

Walker was an assemblyman from Hicksville and heads the Hicksville Republican Club and the Hicksville Republican Committee, which became a fundraising powerhouse under his leadership.

Walker also was Mangano's campaign manager and earlier was an aide to Venditto and was the town's deputy parks commissioner. Walker's late father, John, was a Hicksville-area Republican leader and his mother, Rose, serves on the Nassau County Legislature.

Attorneys for Dean and Adam Skelos have not returned calls seeking comment. James Margolin, a spokesman for the Southern District, declined to comment, as did Nellin McIntosh, a spokeswoman for the Eastern District. Marta Kane, a spokeswoman, for the Town of Oyster Bay, also declined to comment.

Talk at officer's funeral

The Skeloses have pleaded not guilty to charges of conspiracy, extortion and bribe solicitation, in actions allegedly focusing on shaking down companies to obtain jobs for Adam in return for political favors from his politically influential father.

One of those companies was AbTech, an Arizona antipollution company that got a contract from Nassau County.

According to Southern District prosecutors, Dean Skelos, Mangano and Walker went to Brooklyn in January to attend the funeral of New York City Police Officer Wenjian Liu, who had been killed in the line of duty.

Outside the funeral home, Skelos asked Mangano and Walker about county payments to AbTech, the prosecutors said.

After speaking with another Nassau official, Walker "informed Dean Skelos that the payments would be made and took steps to expedite the payments due to Dean Skelos' official position," according to federal prosecutors.

Around the same time, court papers state, Adam Skelos called his father, who assured him, "All claims that are in will be taken care of."

Adam Skelos also was wiretapped telling an informant that if the funds were withheld, "I tell you this, the state is not going to do a [expletive] thing for the county. Any favor that [County Executive Edward Mangano] calls and asks for, it's not happening."