A federal investigation into Nassau’s chief deputy county executive, Rob Walker, centers on a $12-million county storm cleanup contract won by a company that gave money to Walker’s political committee just as the agreement was finalized, according to a source with knowledge of the probe.

Testifying last week in the federal corruption trial of state Sen. Dean Skelos (R-Rockville Centre), Walker confirmed that the U.S. attorney office for New York’s Eastern District is probing his role in “county contracts given to some of your political campaign contributors,” as prosecutors phrased it in court.

Walker provided no specifics about the cases. But the source said Wednesday that Walker was referring to a county contract awarded in 2013 to VIP Splash Waterways Recovery Group Inc., of Island Park.

The agreement, originally for $4 million, was increased to $12 million in 2014. The amendment had been approved by the county legislature, and was executed by the county executive’s office, by Walker’s signature, on the same day that VIP Splash gave $2,925 to Walker’s Hicksville Republican Committee, according to state election records.

Newsday reported the contribution and its timing in March, noting that VIP Splash had incorporated weeks before Nassau solicited bids for the federally funded contract, which covered superstorm Sandy debris cleanup.

The Nassau district attorney’s office also is investigating.

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Walker has not been charged with any crimes. His attorney, Brian Griffin, said Wednesday that Walker “has done nothing wrong, has nothing to hide, and as such, he will continue to provide full and complete answers, documentation and information regarding any and all county contracting issues.”

Federal prosecutors in the Skelos trial called Walker to testify about his role in another county contract that is central to the case against Skelos and his son, Adam. Walker received immunity for anything said during his testimony, and confirmed that he received no promises from the government that he won’t be prosecuted for unrelated cases.

Asked about those unrelated cases — including the contracts for political donors, as well as what prosecutors called “helping someone you had a personal relationship with obtain contracts with entities with business before the county” — Walker said, “I’m ready to answer any questions that they may have.”

With Celeste Hadrick