Third-grade detectives make roughly $11,000 less annually than second-grade detectives, and $27,000 less than first-grade detectives. After retirement, those differences can swell to hundreds of thousands of dollars more in lifetime police pensions for first-grade detectives.

In testimony last year in an unrelated case, Lt. Bernard Whalen, who works in the Police Department’s Office of Labor Relations, said the police commissioner at the time, William J. Bratton, knew the detective promotions process should be more transparent.

“He realizes that people are frustrated about the promotion process to detective second-grade in particular, and first-grade,” Lt. Whalen said. “They’re trying to come up with a way to let people know what’s expected of them.”

For now, though, detectives say the system remains inscrutable. Sergeants make a list of who they want promoted, and it works its way up a ladder of supervisors, with names being knocked off at each rung, until a chief and then a deputy commissioner cull the list a final time and send it to the police commissioner’s office. Unlike the promotional paths to becoming a sergeant, lieutenant or captain — all of which rely heavily on exams — detectives move up at bosses’ discretion, and in most units are never told exactly the rationale. The list, called the “grid,” is confidential.

Commissioner Byrne said the system worked because skills like conducting interviews and running down leads cannot be measured by an exam. Detectives, though, say the secrecy turns them against each other and makes some investigators overly aggressive with cases. They say some are promoted for good cases and others because they are sergeants’ drivers or chiefs’ drinking buddies.

A detective assigned to the “rap unit” at its founding, William Courtney, who had earlier built a major investigation of the record label Murder Inc., said they tried to predict and deter violence at events like celebrity basketball games and rappers’ appearances at shoe stores. He said that put it on the sidelines of a division focused on terrorism.