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Nassau County Police Department’s top cop resigned Thursday following an investigation by the county district attorney’s office into allegations of a politically-motivated arrest during campaign season involving the race for the county executive’s seat, officials said.

Police Commissioner Thomas Dale’s sudden resignation came after Nassau District Attorney Kathleen Rice sent a letter to County Executive Ed Mangano citing “questionable influence” within the department, Mangano said in a statement. Dale, who was appointed by Mangano to police commissioner nearly two years ago, tendered his resignation after meeting with Mangano on Thursday.

“District Attorney Rice today brought a troubling matter to my attention regarding questionable influence within the Nassau County Police Department,” Mangano said. “Upon further investigation, the District Attorney found no evidence to indicate criminality but the investigation itself indicates a fresh look at internal procedures is warranted.”

Mangano’s Deputy County Executive for Public Safety Victor Politi was named acting-police commissioner in Dale’s place.

Another top NCPD official, Chief of Detectives John Capese, has since retired and has been replaced by Narcotics Vice Squad Inspector Kevin Smith, Mangano said.

The incident sparking the shakeup involves the arrest of Roosevelt resident Randy White, who testified in a civil proceeding that former Freeport Mayor Andrew Hardwick’s county executive campaign on the upstart third-party We Count line paid White to gather nomination signatures using a pay-per-signature formula, in violation of New York State election law. Hardwick has denied paying for signatures.

Days after White testified, Hardwick’s attorney unsuccessfully tried to introduce an audiotape of White allegedly saying that he had been paid hourly in an attempt to discredit the witness. Gary Melius, the owner of Oheka castle in Huntington who donated $23,000 to Hardwick’s campaign, then called Dale to request a perjury charge be filed against White, according to Rice.

Dale ordered Capece and a police department attorney to meet Hardwick’s attorney and campaign staff at the First Precinct station house, where Dale told Hardwick’s staff to file the complaint based on the audiotape, but investigators found the tape to be inaudible and declined to file charges against White, according to Rice.

But, police did learn at the time that White had an outstanding warrant for failing to pay a $250 fine for a misdemeanor counterfeiting conviction, which prompted Dale to order White’s arrest, according to Rice. Nassau police detectives arrested White on Oct. 5 after pulling over a public bus he was riding, according to Rice.

While White was being processed at police headquarters in Mineola, off-duty Nassau police Sgt. Sal Mistretta served White with a civil subpoena drafted by Hardwick’s attorney ordering White to appear in court for the election case the following Monday, according to Rice.

Rice, a Democrat, said that she had no reason to believe that Mangano, a Republican, had prior knowledge of the incident, which she termed “inappropriate.” She also said Dale had no prior knowledge and that the commissioner’s involvement does not constitute criminal conduct. But, she added, the investigation is continuing into Mistretta, who also recently retired.

“We’re shocked by these events,” said Mike Florio, spokesman for Democratic minority in the Nassau County Legislature. “And these are serious allegations. This is a serious action. Dale deserves to be fired for this kind of misconduct. It’s a complete abuse of his power.”

James Carver, president of the Nassau County Police Benevolent Association, the union that represents the department’s rank-and-file, said the move should not reflect badly on the rest of the police officers.

“Commissioners come and go,” Carver said. “The one constant is the great work that the men and women do on the street protecting the citizens of Nassau County.”

Dale’s firing comes a month after Mangano fired County Attorney John Ciampoli and less than a year after a ex-Second Deputy Nassau Police Commissioner William Flanagan was convicted of misconduct for quashing his friend’s son’s burglary arrest. Flanagan is appealing that conviction.

Police declined to comment. Hardwick did not return a call.