PHILADELPHIA – Baltimore, Philadelphia and other cities in a bloodstained corridor along the East Coast are seeing a surge in killings, and one of the most provocative explanations offered by criminal-justice experts is this: not enough new immigrants.

The theory holds that waves of hardworking, ambitious immigrants reinvigorate desperately poor black and Hispanic neighborhoods and help keep crime down.

“New York, Los Angeles, they’re seeing massive immigration – the transformation, really, of their cities from populations around the world,” said Harvard sociologist Robert J. Sampson. “These are people selecting to go into a country to get ahead, so they’re likely to be working hard and stay out of trouble.”

It is only a partial explanation for the bloodshed over the past few years in a corridor that also includes Newark, N.J., and Boston, but not New York City.

Homicide detectives, crim-inal justice experts and community activists point to a confluence of other possible factors.

Among them: a failure to adopt some of the innovative practices that have reduced violence in bigger cities; the availability of powerful guns; and a shift in emphasis toward preventing terrorism instead of ordinary street crime.

Philadelphia is losing one resident a day to violence, recording 196 homicides through the third week of June. That is slightly ahead of the total at this point in 2006, a year that ended with 406 homicides, the most in almost a decade. On the first day of summer alone, six people were killed in Philadelphia in three street shootings.

In Newark, the homicide toll has soared 50 percent in four years, from 68 in 2002 to 106 in 2006. Baltimore had 140 slayings as of June 10, up from 122 the same time last year. Boston had 75 homicides in 2005, a 10-year high, and 75 in 2006. So far this year, there have been at least 30 slayings.

Some cities “never bothered to institute the reforms, policies and programs that impacted violent crime because they felt immune from what they saw as big-city issues,” said Jack Levin, director of the Brudnick Center on Violence at Northeastern University in Boston.

These efforts include limiting gun purchases, suing rogue dealers and deploying officers more strategically, based on data analysis.

The vast majority of U.S. homicides involve guns. And they are more powerful than ever. The weapons of choice are semiautomatics that can spray dozens of bullets within seconds.

“We’re seeing 40, 45 shots,” said Richard Ross, Philadelphia’s deputy police commissioner. In one recent killing, “I think they fired 20 shots into him. That’s remarkable.”

Some cite a drop in federal aid for ordinary law enforcement in favor of homeland security spending. According to Ross, federal grants used mostly for police overtime in Philadelphia fell from more than $4 million in 2002 to about $1 million last year.

The number of police officers per capita has fallen 10 percent since 2000 in cities of more than 225,000, according to Northeastern University criminologist James Alan Fox. Yet post-Sept. 11 fears, especially in Boston, have forced police to monitor government buildings and transportation hubs while also watching for street crime, he said.

“Cities that have heavily concentrated and segregated African-American poverty are the places that have increases in homicide,” Sherman said. “The places that have lots of immigration tend not to have nearly as much segregation and isolation” of poor blacks.

Sherman acknowledges the theory is evolving and unproven.

He said immigrants “change the spirit” of a community and affect the way young black men in poor areas relate to each other.

“It seems a plausible way to account for the big difference in the trajectory of homicides” in stagnant cities versus ones with lots of immigration, he said.

The percentage of foreign-born residents is 11 percent in Philadelphia, compared with 22 percent in Chicago, 37 percent in New York and 40 percent in Los Angeles, according to 2005 census figures.

Associated Press writers Ben Nuckols in Baltimore, David Porter in Trenton, Erin Conroy in Boston and Michael Rubinkam in Philadelphia contributed to this report.