AURORA | A year after seeing crime dip — and after several recent years that saw crime plummet in Aurora — crime in Aurora climbed in 2015.

The city’s crime spike, which includes a 12-percent jump in violent crime, mirrors a national trend that has police and experts looking for answers.

“We can’t always explain why there are increases and decreases,” said Aurora police Division Chief Vanessa Wilson.

Some pundits have pointed to a supposed “Ferguson effect” as the cause for increased crime around the country. The theory posits that after the 2014 shooting of Michael Brown in Ferguson, Mo., and the subsequent backlash toward law enforcement, police took a less aggressive approach to crime fighting which led to increased crime.

The theory gained some attention in October when FBI Director James Comey said during a speech that there was a “chill wind” facing law enforcement after Brown’s slaying and said rising crime rates could be connected to that.

But a study from the University of Colorado last week said the theory is largely a myth and pointed out that crime rates in major cities have remained largely flat post-Ferguson.

Aurora’s preliminary statistics, which go through Dec. 27, 2015, show major crime climbed 4.5 percent last year. Murders jumped 83 percent, from 12 to 22, and sexual assaults were up 23 percent. The only crime that dropped compared to 2014 was burglary, which fell 12.7 percent citywide.

Aurora police spokeswoman Officer Crystal McCoy said the department will release final statistics from 2015 sometime by early March.

Wilson said in the case of murders, 22 is in line with historic totals, but 2014 saw an unusually low number of murders in Aurora with just 12.

The preliminary statistics also show that while crime rose in 2015, Aurora police made fewer arrests than in previous years. Arrests were down 5 percent, from 12,613 in 2014 to 11,935 last year. Summonses were down almost 10 percent, from 8,272 to 7,464.

Wilson said it is difficult to pinpoint why there were fewer arrests last year because so many factors play into whether officers can make an arrest.

But, Wilson said, the department had more attrition last year among sworn officers than is typical, and the department has stressed the importance of engaging the community under new Chief Nick Metz, who took over in March.

That means patrol officers are visiting with churches and community groups more and also working closely with crime victims. In years past, the department’s Police Area Representative officers — the beat cops who focus on a particular neighborhood — led those efforts, but Wilson said community engagement is now a department-wide focus.

“Reaching out to the community and being involved in the community is not just a PAR job anymore,” she said.

While the decrease in arrests could be a sign of less-aggressive police work, David C. Pyrooz, an assistant professor of sociology at CU-Boulder and the author of the report on the supposed Ferguson effect, said that isn’t necessarily the case.

Many factors — including witness cooperation -— go into whether police make arrests, Pyrooz said, and just looking at total arrest statistics doesn’t tell the whole story.

If a department sees a dip in arrests for public order crimes such as drinking in public or disorderly conduct, that could be a sign of less-aggressive police work, but without drilling down into specific cases he said the numbers don’t tell much of a story.

“You just can’t pinpoint it at least form the data I’ve seen,” he said.

Pyrooz said annual crime stats rarely tell a story of a larger trend in crime and said such trends take several years to play out.