NEW YORK - An undercover detective's wire has become his undoing.

Prosecutors allege that Sean Johnstone - already accused of unwittingly recording himself using racial slurs - also was caught on the tape describing how he had tampered with drug evidence.

As a result of the tape, Johnstone and a second New York Police Department narcotics officer, Julio Alvarez, were arrested yesterday and arraigned in criminal court in Brooklyn on false-document and official misconduct charges.

Johnstone, 34, and Alvarez, 30, were represented by union lawyers. They were released without bail and ordered to return to court Feb. 6.

Detectives' Endowment Association President Michael Palladino said Johnstone, a six-year veteran, had "an unblemished record" with the NYPD.

"I only urge people not to rush to judgment until he has his day in court," Palladino said.

A spokesman for the Patrolmen's Benevolent Association declined to comment.

Authorities said the corruption case stems from a routine bust by the accused officers of a drug suspect on Sept. 13 in Brooklyn. Following the arrest, Alvarez prepared paperwork claiming they had recovered "17 ziplock bags of cocaine," according to a criminal complaint.

The complaint said that the next day, during a recorded conversation with an unnamed officer, Johnstone confided that 28 bags had been confiscated. He boasted that he and Alvarez "had intentionally only vouchered 17 ziplocks of cocaine from that amount," the complaint added.

Investigators said Johnstone apparently didn't realize a hidden recording device he was wearing was switched on. When his superiors reviewed the tape, they said, they uncovered the misconduct.

On the tape, Johnstone, who is white, and the unnamed officer used the N-word, said police spokesman Paul Browne. That resulted earlier this year in the suspension of the pair.

Internal-affairs investigators still were trying to determine if any drugs were missing and, if so, what happened to them, Browne added.

Court records show felony charges against the drug suspect were dismissed, though he still faces misdemeanors.

If convicted, Johnstone and Alvarez each could face up to 4 years in prison. *