The trend continues to puzzle some criminologists but has reinforced the view of many in law enforcement that factors other than the economy determine the rise or fall of crime.

Los Angeles Police Chief Charlie Beck said crime rates are determined largely by how well police do their job and the "informal social standards" set by communities — that is, what kind of behavior people are willing to tolerate from others.

"The driving forces on crime," Beck said, are " 'What is the likelihood the police will catch you?' and, 'What would your mother or neighbor think if they knew what you were doing?' "

A spokesman for Sheriff Lee Baca took a similar stand. "Communities seem to be banding together to fight crime," Steve Whitmore said. "We can't take the complete credit."

The city, along with much of the rest of L.A. County, finished the year with thousands fewer serious crimes than in 2010, according to preliminary statistics gathered by the Los Angeles Police Department and the county Sheriff's Department.

Homicides, an important bellwether for violence levels in general, finished the year nearly even with the historically low rate of killing that Los Angeles reached in 2010. The LAPD tallied 298 killings in 2011, making it the second consecutive year the city experienced fewer than 300 homicides. It is a benchmark that was unimaginable amid the gang violence and crack cocaine epidemic of the 1990s, when the homicide rate was four times as high.

Throughout the rest of L.A. County, which is patrolled by the sheriff and individual cities' police departments, there were 283 homicides — a 12% decline from the previous year — according to a Times analysis of coroner's data.

Other categories of violent and property crime, meanwhile, continued the downward trend they have followed for the last several years. The number of reported robberies, aggravated assaults, burglaries and auto thefts in the city through Dec. 24, for example, was down between 3% and 9% compared with the same period the previous year, LAPD statistics showed.

The Sheriff's Department, which patrols numerous small cities and the county's unincorporated areas, also posted declines in a preliminary count through November. Serious violent crimes were down 13.5%, and property-related offenses dropped about 2%. The LAPD is scheduled to release its final crime numbers Thursday.

"It is deeply puzzling," said Richard Rosenfeld, a leading criminologist at the University of Missouri, St. Louis. "During past economic recessions, with high unemployment and stagnant incomes, we saw increases in crime. That has not been the case this time."

Since 2007, the last full year before the onset of the country's ongoing economic woes, Angelenos and others in the region have been told to brace for an anticipated surge in crime that has never come.

To the contrary, the region has watched as a downward trend in crime that began nearly a decade ago has continued largely unabated, despite high unemployment, a horrible housing market and cuts to public services. This will be the ninth consecutive year of falling crime in Los Angeles.

Not yet ready to altogether abandon the long-held belief that people's financial well-being is inexorably linked to crime rates, Rosenfeld nonetheless acknowledged that he and other researchers are running out of places to find where that link exists.

Researchers and police have long butted heads trying to make sense of what factors influence crime rates. Police argue that their work is the linchpin, while academics look for larger societal explanations.

The declines of the last several years, experts say, are not easily explained by one factor and are almost certainly the result of several developments and issues combined. More effective crime-fighting strategies; strict sentencing laws that, until recently, increased the number of people in prison; demographic shifts; and sociological influences are likely factors.

Beck and other law enforcement officials have warned that the state's recently enacted plan to address severe prison overcrowding will place more offenders on the streets and increase crime this year.

The 2011 statistics translate into tangible changes in the quality of life in many communities. Compton, for example, recorded 17 killings through Tuesday, a nearly 60% decline from 2007, according to a Times analysis of homicide data. Inglewood, Boyle Heights and Lancaster also posted significant reductions in bloodshed. In total, there were about 15,000 fewer crimes reported in 2011 than the previous year in areas patrolled by the LAPD and Sheriff's Department, according to the agencies' figures.

By far the most dramatic drop in Los Angeles and sheriff-patrolled areas was in the number of reported rapes, which fell about 20%. The decline may be due in part to advances in the use of DNA evidence to identify and jail serial rapists, said Gail Abarbanel, director of the Santa Monica-UCLA Rape Treatment Center, and Beck. Abarbanel, however, cautioned against making too much of the numbers, since only a fraction of rapes are reported to police.

The year in crime was not all positive, as the region was dotted with pockets that bucked the overall trend. Some examples: The LAPD's Central Division, which includes downtown, experienced a jump in homicides, robberies and burglaries; the Harbor Division, which includes San Pedro and other neighborhoods around the mammoth port, saw a disturbing rise in killings and gunplay; and the Sheriff's Century station saw a surge in homicides and auto thefts.

Beck downplayed such trouble spots, saying they are inevitable in a county with a major urban center and nearly 10 million people. Crime, he said, needs to be tracked over several years for a meaningful sense of a neighborhood's progress or decline. "You're always going to have a gang feud flare up somewhere or a criminal enterprise that takes time to shut down," he said.

But Beck expressed concern and puzzlement over the 168 gang-related killings committed in the city through Saturday — a 6% increase over the same period the previous year. Oddly, the main precursors to gang homicides — the number of shooting victims and gunshots fired — and gang-related crimes in general were all down.

Efforts by the LAPD to tamp down crime last year were marked by a stark increase in the number of times officers fired their weapons. The 61 officer-involved shootings through Saturday represent a 65% rise over the same period the previous year and more than in any other recent year, department figures show. The LAPD's homicide count does not include the 25 people killed by officers in 2011.