In a year in which the number of homicides plummeted across California, a few communities were outliers in 2013, leaving crime experts puzzled and some residents on edge.

In Pomona, a city of 150,000 people on the eastern edge of Los Angeles County, killings jumped from 17 to 29, a 70% increase, according to records released Thursday. Communities patrolled by the Riverside County Sheriff's Department recorded a homicide increase of more than 40%.

While overall crime is down in areas covered by the Long Beach Police Department and the Los Angeles County Sheriff's Department, homicides remained relatively flat in 2013. Long Beach recorded two more killings and the sheriff one more compared with 2012.

HOMICIDE REPORT: A chronicle of L.A. County homicide victims

The numbers are all the more striking when compared with cities like Los Angeles, where homicides dropped to the lowest level since 1966. Even traditionally crime-plagued areas did well in 2013: Killing in Oakland dropped from 131 to 92, while Stockton saw homicides plummet by half to 32.

Criminologists said that upticks in Pomona and Riverside County are hard to explain, but that it is far too early to declare any major trends. In smaller communities, localized issues such as gang disputes and reduced police patrols can result in more homicides. Often it can take time for police to create strategies that bring down the violence, they said.

"Homicides are often just a matter of inches about where a shot goes," said James Alan Fox, professor of criminology at Northeastern University.

Robert Weisberg, who heads the Stanford Criminal Justice Center, said the jump in homicides should prompt a rethinking of how police patrol those areas.

"Both of those increases — they're humongous," he said. "Nationally the trend is either flat or down, which makes it even more anomalous."

The rise in killings has rattled Pomona perhaps more than any other place in Southern California. The city has dealt with gang problems for years, but killings over the last two decades have steadily dropped.

Pomona recorded only 11 homicides in 2011, and 17 in 2012. Officials say the 2013 total of 29 is the highest since the 1990s.

"We cringe every time there is homicide," said Pomona Police Capt. Michael Olivieri, who joined the department a year after the city saw a record 44 homicides in 1989. "We are not pleased. It is not acceptable. We believe we're going to suppress it."

Two factors appear to be contributing to Pomona's problems: Violence between young people from warring street gangs and a depleted police force.

According to the city, the number of sworn officers in the department has declined by about 50 since 2010 due to budget cuts.

Police attribute many of the homicides to rising gang violence in certain sections of the city.

The department has been struggling to come up with an effective strategy. Police gang task forces meet every other month to go over gang intelligence, Olivieri said, and plainclothes officers routinely flood the streets to catch criminals in the act. On at least one occasion, he said, a suspect tried to carjack a plainclothes officer.

"There's a conflict out there," he said. "And we're addressing it."

Resident Larry Edwards said the violence is hard to miss.

"Down that way, one guy got shot in the middle of the street," Edwards said, looking down Angela Street. "And down the way, around the corner …"

Edwards has owned a four-unit building in the neighborhood since 1977, and called the violence "cyclical." He recalled four shootings in the area in 2013, including a teenager killed in an alley 100 feet from his building.