An undercover Nassau narcotics detective is under investigation after authorities say they learned that a confidential informant he paid to do a drug buy in Oceanside wasn't involved in the transaction.

The district attorney's office is contacting attorneys for defendants who have open cases in which Det. Michael Cipullo was involved, and Cipullo has been placed on desk duty, authorities said.

In a February letter obtained by Newsday, a prosecutor explained to a defendant's attorney the circumstances of the 2014 case that led to the probe.

Without naming the defendant in the unrelated case, Assistant District Attorney Teresa Aiello said Cipullo wrote in his report that an informant bought 20 Xanax pills from a defendant on Nov. 28, and filled out a receipt saying the informant was paid $50 for the work.

But Cipullo "has since indicated to our office that no C.I. was involved in the alleged purchase," and he "in fact purchased the Xanax himself, although he did pay the C.I.," according to the letter.

In an audio recording of the alleged drug transaction, "there appears to be no reference" to an informant being present, the letter also states.

Shams Tarek, a spokesman for acting District Attorney Madeline Singas, said prosecutors are still determining the scope of the matter. He said he didn't know how many notices have been sent out.

"The goal of all prosecutions is fairness and justice," Tarek said. "We determined that the nature of Det. Cipullo's conflicting statements should result in immediate transparency to defendants."

When asked if prosecutors also would make notifications in cases that are closed, he said: "The office is reviewing the matter."

The case that sparked the inquiry is open, but at least one defendant in a different case involving Cipullo has pleaded guilty since hearing of the detective's conflicting statements, Tarek said.

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Police spokesman Insp. Kenneth Lack said Cipullo has been transferred to a central detective squad and assigned to administrative duties pending the outcome of the investigation.

Cipullo is a 25-year veteran who earns a base salary of $125,600, records show.

Glenn Ciccone, president of the Nassau County Police Department Detectives Association, called Cipullo "a highly decorated, phenomenal undercover narcotics detective." He said he could see "where there could be an administrative mistake here and there" when it came to handling "buy" money.

"I do know it's not black and white, what they have to do over there," he said of narcotics detectives.

Dana Grossblatt, president of the county Criminal Courts Bar Association, called for an investigation into all of Cipullo's cases -- not just the open ones.

"How on earth could it be an administrative error to say a C.I. made the buy -- oops, I made the buy?" she asked. "If one person is sitting in jail based on an untruth on his part, it's one too many people."