With fewer people on the streets, and more in their homes, some big U.S. cities saw significant decreases in crime during the first month of the pandemic shutdown, according to statistics from 30 large and midsize cities and counties gathered by the Police Executive Research Forum. Some saw spikes in violent crime and auto theft, however, and police said closed businesses were more frequently targeted for burglaries.

The Washington-based think tank compared crime statistics from March 16 to April 12, roughly the outset of the coronavirus shutdown in America, with the same period in 2019. Of the 30 jurisdictions, 18 saw decreases in violent crime — murder, rape, robbery and aggravated assault — as the pandemic hit the United States, which included a 33 percent drop in San Francisco, a 25 percent drop in New York and a nearly 25 percent decline in Los Angeles.

Washington and Baltimore both saw an 8 percent decrease in violent crime. Prince George’s County, Md., the only suburban Washington jurisdiction in the study, experienced a 24 percent drop in violent crime in the month after the coronavirus crisis struck. Overall, the 30 cities and counties surveyed experienced a combined 11.5 percent drop in violent crime, and 23 percent drop in property crime.

But 12 cities saw increases, which included a 21 percent jump in Denver and a nearly 12 percent increase in Houston. Austin and Nashville were among the cities that saw smaller rises in violent crime.

Homicide numbers were mixed — deaths increased in nine cities, decreased in nine cities, and 12 reported no change. Slayings in Los Angeles dropped from 31 during that period in 2019 to 16 in 2020, but homicides in Nashville during that period rose from four to 14. Homicides in Baltimore rose from 20 in those weeks last year to 23 this year. In Washington, they went down, from 11 to 10. Overall, the total number of homicides in the 30 jurisdictions dropped from 224 to 216, which is a 3.5 percent decline.

Chuck Wexler, the executive director of the research forum, said he and a number of police chiefs he had spoken to think “the pandemic has not dramatically altered traditional patterns of gang warfare, drug-related violence, and individuals using guns to settle personal disputes. These serious, deeply entrenched problems continue to drive much of the violence in our communities.”

Aggravated assaults increased in 13 of the 30 jurisdictions, with increases of at least 25 percent in Washington, Denver, Houston and Volusia County, Fla. Overall, aggravated assaults declined 8.5 percent in the 30 jurisdictions.

Crime numbers, particularly homicides, can randomly fluctuate and are best assessed over longer periods to detect true trends. But the first month of the pandemic created unprecedented changes in American society, and it remains to be seen whether some of the dramatic crime shifts in that month continue during the stay-at-home period and beyond.

Property crimes, for example — burglary, larceny and auto theft — declined dramatically, with 25 of the 30 jurisdictions reporting drops in the March-April period this year. Baltimore saw a 43 percent decrease, Washington a 36 percent decrease and San Francisco a 46 percent decrease. Larcenies dropped in 28 of the 30 jurisdictions, the forum’s data show.

It figured residential burglaries would plummet, as more people were staying home during the day. But Wexler said police chiefs report business burglaries are surging as thieves target shuttered establishments and fewer cleaning crews are working in office buildings at night. He said commercial burglaries drove the overall burglary rate up nearly 44 percent in Seattle, 41 percent in Denver and 17.5 percent in New York. Total burglaries fell 23 percent in Washington and 36.5 percent in Baltimore.

Another side effect of the pandemic — people not driving their cars nearly as much — may have contributed to some spikes in auto theft. Auto thefts increased in 16 of the 30 jurisdictions, including a 59 percent rise in Austin and a nearly 26 percent rise in Salt Lake City. Auto thefts in Baltimore dropped nearly 35 percent, and the District saw a 2.5 percent drop.

Police have been less busy during the pandemic, the statistics show. Twenty-nine of the 30 jurisdictions reported declines in calls for service. Only Prince George’s County, with a 3.4 percent rise, showed an increase, and Chicago saw a 25 percent drop in calls. Washington and Baltimore saw approximately 20 percent fewer calls for service.

Arrests plummeted, too, as police joined the effort to incarcerate fewer people during the outbreak. Only 22 jurisdictions provided arrest data for the month, but 18 were down for Part I crime; for lesser Part II crimes, arrests were down in all reporting jurisdictions. Boston police arrested 66 percent fewer people for serious crimes, while authorities in Miami and Chicago arrested 61 percent fewer people and 53 percent fewer people, respectively. Washington saw 44 percent fewer Part I arrests, and Baltimore had 36.5 percent fewer Part I arrests.

Part I crimes are defined by the FBI as murder, rape, robbery, aggravated assault, burglary, larceny and auto theft. Part II crimes include simple assaults, fraud and forgery, weapons violations, sex offenses and drug cases.

Wexler said police officials wonder whether the drop in arrests, as well as a pullback on community policing because of social distancing, will eventually lead to more crime. Traffic enforcement has been scaled back dramatically, Wexler said. In New York City and the state of California, police have expressed frustration about repeat offenders being released back to their communities, where they could possibly swiftly reoffend.

Police are also on alert for increases in crimes related to the pandemic’s effect on unemployment, family financial troubles and domestic violence. “That doesn’t mean that the factory workers or retail clerks who lose their jobs today will become the burglars or bank robbers of tomorrow,” Wexler said. “But the desperation that comes with this level of economic hardship could impact domestic violence, child abuse and other types of crime.”

The cities and county police departments surveyed by PERF were: Atlanta; Aurora, Colo.; Austin; Baltimore; Boston; Charleston, S.C.; Chicago; Dallas; Denver; Detroit; Grand Rapids, Mich.; Houston; Irving, Tex.; Los Angeles; Miami; Multnomah County, Ore.; Nashville; New Rochelle, N.Y.; Newark; New York City; Prince George’s County, Md.; Salt Lake City; San Diego; Seattle; San Francisco; Syracuse, N.Y.; Tempe, Ariz.; Vancouver, Wash.; Volusia County, Fla.; and Washington, D.C.

This post has been updated.