It's called the "blue wall," the unwritten code that says police officers should keep silent about the misdeeds of fellow officers -- no matter the cost.
But police experts believe that a recent Essex case in which officers turned in a colleague on charges of brutality could be a sign that the wall is crumbling.
These days, some officers are willing to report one of their own to protect the image of the department or protect themselves, the experts say. Another motivation: improving the image of police, in the wake of the Mark Fuhrman tapes released in the O. J. Simpson trial and of the Rodney King case.
"It makes police officers feel they've got to go the extra mile to prove themselves," said Jack McDevitt, director of the Center for Criminal Justice Policy Research at Northeastern University in Boston.
Still, experts say, it's hard to gauge how often police report misconduct.
"There are no firm statistics that exist on how many incidents of brutality or misconduct occur and how many are really observed . . .," said Tony Narr, a senior research associate at the Police Executive Research Forum in Washington. "You read routinely about the blue wall. The degree to which that really exists depends on the precinct or department.
"Has it been tolerated in the past? Is is tolerated now? Has it improved over the years? We can't say for sure."
In the Baltimore County incident, Officer William R. Goodman Jr., 34, was indicted and charged under Maryland's hate-crime statute, accused of uttering racial epithets while punching and kicking a suspect at the Essex precinct in July. He was also indicted on misdemeanor charges of battery and misconduct in office.
An investigation into the incident was triggered by fellow officers and supervisors who witnessed it, police officials say.
"If these officers are willing to come forward and testify -- then yes, you do have an unusual case that speaks well for the police department," Mr. McDevitt said.
Some police departments have been bedeviled by high-profile cases involving officers. For example, 16 New York City officers were indicted in May on brutality, perjury and theft charges; nine New Orleans officers were indicted in December on federal weapons and drug charges.
But other departments are making strides to ensure that misconduct will not be tolerated -- to knock down the blue wall.
"The nature of police work itself, the danger involved, the risk, it separates [officers] from the normal linkages to friends and family," said Hubert Williams, president of the Police Foundation in Washington.
"They tend to become a close, tight-knit group. When you come into that kind of environment, you have to play by the rules, meaning you don't 'rat on a fellow brother officer.' "
Lt. Walter E. Doyle III says that in his 3 1/2 years as an investigator in the Baltimore County police internal affairs section, most officers have been willing to speak up when they saw something wrong.
From January to August this year, 289 complaints were filed against county officers, ranging from reporting late for duty to serious misconduct such as brutality, according to county statistics. Among the cases being investigated by internal affairs, 174 complaints -- 60 percent -- were filed by members of the police department.
"Times are generally changing . . . " Lieutenant Doyle said. "I think if it's something serious, they'll speak up, but the gray areas where it's not clear whether something wrong happened, they tend not to know what to do. But I think most officers here, because of their professionalism and values, won't tolerate anything seriously wrong."
Recruits, who go through eight months of training, are "indoctrinated early on with the county's values system of integrity, fairness and honesty. Those are the three watchwords to keep in mind when policing here," said Capt. Brian A. Uppercue, a county police spokesman.