The Nassau district attorney has subpoenaed the county for documents related to contracts awarded to former NYPD Det. Richard "Bo" Dietl to study a police merger and a treatment plant's security, sources close to the investigation said Tuesday.

The June 26 subpoena from Acting District Attorney Madeline Singas seeks from the Nassau County Police Department and the county's Department of Public Works all documents, reports, correspondence, invoices, receipts and any logs related to the contracts, according to a source.

The district attorney's office did not comment Tuesday on the investigation. Last month, Singas said she would review Dietl's contract with Nassau County to study a possible merger of the Freeport and Hempstead village police departments with the county force.

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Both Nassau County Attorney Carnell Foskey and Dietl also declined to comment.

Dietl, chairman and chief executive of the Manhattan-based private investigation firm of Beau Dietl & Associates and a frequent cable news commentator on law enforcement issues, was awarded a $24,000 contract in 2014 to serve as a "public safety adviser." Dietl's firm produced a 13-page report on a merger of the Freeport and Hempstead police departments with the county, a potential consolidation that officials in both villages neither knew about, nor would approve.

In 2011, Nassau's Department of Public Works awarded Dietl a contract for $24,947 for "security consulting services" at the Cedar Creek Sewage Treatment Plant. A DPW spokeswoman has said the county was "satisfied" with the work performed by Dietl's firm -- also a 13-page report.

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Both contracts fell below the $25,000 level that would have required scrutiny from the Nassau County Legislature.

Dietl has been paid for the 2011 contract, county officials have said, but not for the merger contract, which Acting Police Commissioner Thomas Krumpter has called in a statement "incomplete."

County Executive Edward Mangano has said it was never the county's intention to take over the Freeport and Hempstead police departments, but said the county was looking for ways to lower Hempstead's crime. Krumpter was tasked with overseeing Dietl's work, a Mangano spokesman has said.

James Carver, president of the Nassau Police Benevolent Association, has been highly critical of the contracts, saying the work that Dietl's firm performed could have been done by the police department internally.

Carver said Tuesday of Singas: "It appears she's doing her due diligence."

Singas, in earlier comments to Newsday, said Dietl's police department merger report appeared to be a "cut-and-paste" document and that parts of it appear "plagiarized."

Dietl, an Albertson resident, has strongly defended his work, telling Newsday recently: "All I'm trying to do is help the people of our county."