Nassau County prosecutors said Friday they would not file criminal charges in connection with a controversial police department contract awarded to New York City mayoral candidate Bo Dietl.

Dietl, a retired NYPD detective with his own law enforcement consulting firm, was awarded a $24,000 contract to study the possible merger of the Freeport and Hempstead police departments into the county force — a proposal that neither department knew about.

After Newsday reported in 2015 on the contract and a resulting 13-page report, the county refused to pay him, citing “incomplete” work. Dietl also received a $24,947 contract in 2011 from Nassau’s Department of Public Works for “security consulting services” at the Cedar Creek Sewage Treatment Plant.

Brendan Brosh, a spokesman for Nassau District Attorney Madeline Singas said in a statement Friday that although Dietl’s police consolidation study “appeared unnecessary,” no charges would be filed.

“This office investigated the process by which Bo Dietl & Associates received two Nassau County contracts at just below the threshold that required legislative approval, one of which was for a police department consolidation study that appeared unnecessary,” the statement read. “While our investigation concluded without charges, the district attorney has proposed many reforms to Nassau’s contracting process, including the appointment of an independent inspector general to prevent contract corruption and manipulation.”

Brosh declined to answer questions on the reasoning behind the district attorney’s conclusion.

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Dietl, who officially announced his candidacy for mayor at a news conference Tuesday on the steps of City Hall, said he was “complimented” for his work on the treatment center contract, but was never paid for the police contract and is contemplating suing the county.

“How can there be anything criminal when I never got paid?” Dietl said by phone Friday, and chided the district attorney for the probe. “Madeline, I think you got a little bit something else you should be looking at, like these MS-13 gang members in Nassau County. You should be going after them, instead of worrying about Bo getting a contract he never got paid for.”

In a statement, Brian Nevin, a spokesman for Nassau County Executive Edward Mangano, declined to comment on the possible legal action, but said: “The district attorney has questioned his work product and the subject of plagiarism remains unresolved.”

At the time, Singas criticized Dietl’s report, saying it appeared plagiarized, calling it a “13-page cut-and-paste report” and said she would examine the contract as part of her probe into the county’s contracting process that she called “a recipe for corruption.”

Singas called for the creation of an independent inspector general to examine all county contracts — a proposal resisted by the majority of the Republican-led county legislature.

Instead, Mangano hired a county procurement compliance director and instituted a digital tracking system for all contracts. Mangano has pleaded not guilty to unrelated federal corruption charges.

Dietl’s contract was not subjected to initial public scrutiny because it fell below the $25,000 price tag that would have warranted legislative scrutiny.

The Nassau County Legislature agreed to public votes on all contracts above $1,000 after Newsday reported hundreds of pacts were issued to politically connected vendors for amounts just below the approval threshold.