LOS ANGELES — Christopher J. Dorner had a long list of grievances and potential victims, most of them police officers he had once worked with, and he was already suspected of killing the daughter of a retired police official and her fiancé. So law enforcement officials moved rapidly to protect their own, removing officers from the streets and setting up the largest security detail in any city in recent history.

“I’ve not seen anything like this in my time anywhere,” said William J. Bratton, a former police chief in Los Angeles and former police commissioner in New York. “The threats being made against families, the actual commission of murder, that is a line that is not crossed in America. The department had to create a plan of historic proportions.”

Some 50 police department officials each had as many as eight plainclothes officers assigned to protect them and their families by patrolling their neighborhoods — an unprecedented level of protection for such a large group of officers. The focus meant that hundreds of officers were taken off regular duties, leaving some nonemergency calls to go unanswered. And the presence of plainclothes officers transformed life in dozens of suburban neighborhoods, as the police stood guard on porches with shotguns and used dogs to sniff for bombs in backyards.

Most of the officers stayed off the street, forgoing their normal work to protect themselves. Some had their children stay home from school, and several went out of state.