That, by at least one way of reckoning, makes them routine: From 1992 to 2008, nearly 2,000 New York Police Department officers were arrested, according to the department’s own annual reports of the Internal Affairs Bureau, an average of 119 a year.

The rarely seen internal reports were obtained last month by the New York Civil Liberties Union through the Freedom of Information Law. They show that the number of tips logged each year by Internal Affairs has tripled since 1992, a trend that top police officials attribute to an opening up of the process and more diligent cataloging of public response to police interactions — including compliments as well as complaints. The number of investigations pursued over the same period has dropped by more than half, which Chief Charles V. Campisi, who runs the unit, called “the truest reflection of the corruption the department faces.”

Most of those investigations involved drugs, theft or crimes like fraud, bribery or sex offenses, on and off the job. Inquiries in these categories have largely decreased in recent years, but cases involving abuse of suspects have risen significantly. The 2006 report noted the “unprecedented” rise and gave an example: several officers followed a woman wanted for petty larceny into a store and one “struck her in the head with his firearm for no reason.”

“History tells us there always will be bad cops, and the department will never be able to completely control that,” said Christopher T. Dunn, associate legal director of the civil liberties union, who has spent weeks studying the documents. “But what it can control, and what it should be held accountable for, is how it responds to corruption.”

For better or worse, the police are left to police themselves, and the reports, which together total more than 600 pages, provide the most comprehensive record available chronicling that imperfect science. They tell a colorful story not only of officers who betray the badge but also of the little-known agency charged with rooting them out.