There is no such thing as a routine call, most police officers will tell you. Violence can explode anytime, anywhere.

But information about possible dangers can change responding officers' approach.

Officers called for a report of a domestic disturbance might end up as referees between an angry teen and an irritated mother, said Paul McCauley, a criminology professor at Indiana University of Pennsylvania. Or they might end up under fire, like the Pittsburgh police officers killed Saturday in Stanton Heights.

"Domestic violence calls are responsible for the vast majority of injuries [to officers]," said Sgt. Tom Huerbin, of the city's Zone 5 police station in East Liberty. "You answer so many of them. You may get complacent."

Ideally, "police departments would respond to every domestic disturbance call as if they knew they would be confronting an emotionally disturbed person with a machine gun," said Mr. McCauley, a Marine and former police officer. "But then every call would involve a SWAT team, and we can't do that."

When officers have been alerted to red flags on a call -- possible weapons, report of an emotionally disturbed person, previous calls -- they approach differently.

The caution starts when officers approach the scene, said Mr. McCauley.

"You probably don't want to use lights and sirens. They're likely to excite the person," he said. "The officers should be doing things to calm the situation rather than escalating it. When the officers get there, they should probably not park directly in front, because the person could shoot out of the window."

The officers should approach "tactically" -- meaning "they want to spread out a bit, so both don't get hit with a shotgun blast." If possible they'd try to look in a window to see if anyone is waiting. "The officer wants and needs the advantage," said Mr. McCauley.

He said the officers are in a very delicate and dangerous situation when they walk up those steps to a house.

"Most doors, you can shoot through the door blindly," he said.

An option is to ask the person who answers the door to come outside, he said. Or they can ask the person inside to show his or her hands.

Though knowing weapons might be involved could change the way officers approach a situation, it wouldn't necessarily have made a difference in the Stanton Heights incident, said Charles J. Key Sr., a former Baltimore police officer who now is a police training consultant.

"The short answer is that there's no training on earth that will prepare an officer for what these officers faced. Domestic violence calls are dangerous but they are generally dangerous because you can get caught between warring factions in a family. This is well beyond that. This is an ambush."