And it's not just because of job growth in those cities, Roman suggested. Huge surges of immigrants filling those cities have contributed to lowering crime, he said. Immigrants move into blighted areas and create cohesive communities with low crime — contrary to stereotypes that such communities cause societal problems, he said.

"It's about racial and economic segregation," Roman said. "It's about immigration and gentrification."

Chicago, meanwhile, has made national headlines for its gang and youth violence, and last year had the most homicides of any U.S. city. But it also has a population of nearly 2.9 million, and its homicide rate is half that of Baltimore's. Chicago's homicide total this year is down more than 30 percent compared to 2012, and is headed for the lowest point since the 1960s.

Roseanna Ander, executive director of the University of Chicago Crime Lab, said Chicago's numbers are high but get extra attention because of the city's ties to the president, its high-profile mayor and the sheer number of victims.

"Certainly, if you live in many of these neighborhoods, they don't feel safe," Ander said. "But I think a lot of people conflated the largest number with the highest rate, and have been trying to untangle what that means. … It is troubling if people are being shot and being killed, but when you look at the same weekends the previous years, the numbers were the same or lower."

Ander said the city appears to have headed off a sustained increase in gun violence by broadening its approach to include funding for school-based programs for at-risk youth and a private-sector fundraising drive to expand other youth programs.

"If you rely only on the police to suppress and reduce crime, there are other unintended costs," she said, referring to policies that strain relations with the public.

By Wednesday, Rawlings-Blake was frank about the fear in neighborhoods, saying that she was "sad about the state of our communities" and that city residents were killing each other.

In interviews, Batts has given a wide array of reasons for the spike in killings. He has discussed gangs on Kenwood Avenue, where an 18-year-old girl was killed in a quintuple shooting; retaliation related to disputes at nightclubs; and problems spilling over from the city jail.

Batts has regularly mentioned the Black Guerrilla Family gang since arriving here from California in the fall, and has put the group at the center of some of this week's disputes as well. Police have made several arrests in recent crimes, though none of the past week's murders has resulted in arrests yet.

Wexler of the Police Executive Research Forum said days or weeks of flaring violence are not uncommon and police are proving to be better at focusing resources on preventing retaliatory shootings when outbursts happen.

Marc Morial, president of the National Urban League and mayor of New Orleans from 1994 to 2002, said Baltimore's crime is an economic problem that many cities struggle with. One in four residents in Baltimore lives under the poverty line, and the unemployment rate was 10 percent at the end of May.

A major factor is unemployment among youth and young adults, Morial said. "So when you take the illegal narcotics and trafficking in dope, and on top of that you have easy access to guns combined with high unemployment and very difficult economic conditions, it exacerbates the problem."

Joseph Pollini, a professor at the John Jay College, favors aggressive policing tactics like New York City's controversial stop-and-frisk policy. But he added that authorities must focus on community policing as well or face no cooperation from the public.

"Even law-abiding people are going to be against the police because they don't like their kids getting stopped all the time," he said.

At a special meeting of the City Council's public safety committee Wednesday night, several members said they would like to see police more involved with the community.

Pollini said other large cities have benefited from working in lock step with federal officials and building racketeering cases against gangs.

"Once [crime] starts to spike, it becomes more and more problematic," Pollini said. Crime rates have been falling since the 1990s in most cities, he said, and "inevitably it's going to come back up again. To what degree no one really knows, but Baltimore may be an indication of what's going on."

When crime rises, Pollini said, residents feel trapped in their homes and don't want to interact as much with their neighbors. "They're not out there marching in the streets, most of the time people are just apathetic to everything and lock themselves in the house."

The last year in which violent crime rose nationally was 2006, when the rate went up by 1.9 percent. Before that, from 1996 to 2005, violent crime had declined by 17.6 percent, according to F.B.I. figures.