Sitting in a bar-restaurant in Midtown on an overcast afternoon last week, the undercover detective turned author was finishing his second Bloody Mary and telling cop stories from his book, a memoir.

There was the time he went too far, when he dropped a single bullet into the chamber of his five-shot Smith & Wesson to play Russian roulette with an innocent man in the backseat of his patrol car. At the time, the undercover detective, Robert Cea, was under investigation for murder. The New York Police Department suspected him of shooting a drug informant on a roof in Red Hook.

The man in the backseat of the patrol car had not been picked up on suspicion of having committed a crime, but rather on the detective’s hunch he might have known who really killed the drug informant – not Mr. Cea.

Pressing for answers, the detective pushed his gun deeper into the man’s skin, using the barrel to pry the lid of a closed eye open. He then pulled the trigger.

Click. Empty chamber. Who killed Cholito, he wanted to know. The man couldn’t say. The officer pulled the trigger again.

Click. Another empty chamber.

“This wasn’t good cop, bad cop role playing; this was all bad cop, bad cop,” Mr. Cea writes in his first book, “No Lights, No Sirens: The Corruption and Redemption of an Inner City Cop,” which is being published today. The man who murdered the informant Mr. Cea was suspected of shooting in 1989 was a rival dealer who went by the street name “GTO” and was convicted in 1990, Mr. Cea said. The detective was cleared but wrote that his behavior was unjustified.

“I had lost total control; reasoning, logic, any street communicating skills that I had developed over the years vanished,” he writes of the Russian-roulette moment. “I had become as dark as the streets I had patrolled for so long. All of the hatred erupted out of me like a hammer striking the primer of a .45.”

The author said he was in negotiations with producer Albert Ruddy, of the “Godfather” and “Million Dollar Baby,” for the sale of screen rights, but the memoir reads more like a confessional, an indictment of New York’s criminal justice system, than a Hollywood thriller. It also contains information that might prompt convicts arrested by Mr. Cea to sue for wrongful arrest. Mr. Cea said his lawyers advised him that the statute of limitations on his crimes make him immune. Prosecutors said yesterday they were unaware of the book and declined to comment. If defense attorneys come after him now, Mr. Cea said, he would welcome it.

“So what if I stir up a little controversy?” he said. “The law should be changed anyway. The cops need more power under the law to question more people in the street we feel are dirty,” he said. “I never broke the law for personal gain. I broke the law to put animals with guns behind bars.”

Confession: As a police officer from 1982 until his retirement in 1994, Mr. Cea said, he lied on the witness stand in more than 200 criminal cases, to assure convictions. Most of the time, he lied about the probable cause he needed to make an arrest, lies such as testifying he had seen “the bulge” of a gun through a man’s jacket, leading him to frisk the man and then make the arrest.

“I could never see any bulge,” he said.

The practice of lying on the witness stand to assure convictions was so widely known within the department that it had its own name, he writes: “test-i-lying.”

Confession: The detective would often bribe street informants with seized drugs or cash for information that would lead to more serious, sexier arrests. In one case, he writes, he personally cooked up heroin for his informant.

The chief spokesman of the Police Department, Paul Browne, expressed doubts about Mr. Cea’s claims.

“When someone boasts about lying, all other assertions become suspect,” Mr. Browne said in a statement.

Mr. Cea said he wasn’t boasting about lying, but telling the truth.