Rates have fallen for almost every type of crime in San Francisco in the first three months of the year, and city leaders don't know why.

All kinds of violent crime are down by 20 percent so far this year, and homicides have dropped more than 60 percent, according to crime statistics released by the city Thursday. There were 11 homicides reported in the first three months of 2009, compared with 28 in the same time period last year.

Generally, crime rates climb during economic downturns, criminal analysts say. The fact that they've dropped off so far this year could be a sign that new crime-fighting efforts focused in particular neighborhoods are starting to work - or it could just be good luck, Mayor Gavin Newsom said.

"Sometimes these things are just inexplicable," Newsom said. "Last year homicides went up, and I thought we were making a lot of progress."

The Police Department counts 97 homicides in San Francisco last year, not including one person who died at Ocean Beach and whose death is being investigated by the U.S. Park Service, and one person who died in March but whose death wasn't ruled a homicide until this year. The department counts 11 homicides so far this year, including the death from March 2008.

Rapes, robberies and aggravated assault cases all have dropped off this year, by 18 percent to 24 percent. Property crimes - including arson, burglary and auto theft - have fallen 20 percent so far.

Police Chief Heather Fong said one possible explanation for the drop in crime is the city's new "zoned" policing, a program that started in February 2008 and involves focusing police resources in the most crime-ridden communities.

Authorities said two months ago that in the first year of the new policy, homicides dropped just 6 percent in the city overall, but fell 34 percent in the targeted communities - the Tenderloin, Western Addition, Mission, Bayview and Visitacion Valley.

Also helpful, Fong said, is that the city has finally reached full staffing levels, adding more than 400 patrol officers to bring the total full-time policing staff to 1,971 officers. The city has not been able to reach that level since San Francisco residents voted to require it in 1994.

Fong noted that San Francisco hasn't been hit as hard by the recession as have other communities in the United States, which could be a reason crime rates haven't climbed. But she said the department has seen more "get rich quick" scams that single out people who don't speak English.

"It's great to see reductions in crime," Fong said. "Now our work is to sustain that."