Mayor Nutter, top police officials, and prosecutors, along with criminal-justice experts, say the decrease in homicides reflects a new emphasis on data-driven policing, a crackdown on gun criminals, and sweeping reforms in the criminal courts.
In particular, police and prosecutors have targeted so-called hot spots - areas identified as the city's wellsprings for crime.
The fall in homicides reflects a general decline in violent crime. Violent robberies and serious assaults are also down sharply this year. The count of shooting victims has fallen 18 percent, from 633 victims in the first six months of last year to 518 so far this year.
In an interview, Commissioner Charles H. Ramsey cautiously described the half-year numbers as the results of departmental efforts to attack crime in a smarter fashion.
"There has been a general acceptance of trying to think differently about crime," he said.
With sophisticated computer mapping and data analysis, Ramsey said, the department has begun to anticipate crime trends and get ahead of them.
"I've been around for a long time, and what [once] passed for analysis was simply counting crime," he said. "Now we are actually analyzing crime and trying to learn from that analysis ways in which we can be more effective and tweak our strategy."
District Attorney Seth Williams said he viewed the trend as "a sign of hope."
He added, "We're very thankful for that success, and the lives we save are real."
At the current pace, the annual homicide figure would be lower than at any time since 1968.
An Inquirer analysis of city homicide figures over the last 50 years found that the midyear results generally mirror the second half of the year.
If that holds true, Philadelphia would conclude 2013 with roughly 250 homicides, a modern-era low that would fulfill Mayor Nutter's 2008 inaugural promise of cutting homicides up to 50 percent from 2007, when 391 people were killed.
Nutter praised the 2013 trend as the result of the "coordinated and collaborative efforts" of the Police Department, District Attorney's Office, and other law enforcement agencies.
But he expressed caution.
"We are not taking anything for granted," he said. "A half year is a half year. It is not a full year. I am very mindful that we still have to get through the summer and fall. We'll look at this and we'll smile at the numbers for about five minutes and then get back to work."
The steep decline in gun homicides follows concerted efforts by police, prosecutors, and judges to overhaul the city's criminal-justice system.
For police, that has meant shifting away from a reactive approach to a more focused one in which the department tries to anticipate violence and to deploy its officers cohesively to quell it.
Ramsey said the department had benefited from enhanced data-analysis software and was better able to understand and attack crime patterns.
The force, he said, has improved at identifying "the people out there causing the most harm in terms of shootings" - and is proactively working to squelch retaliatory violence.
"You can have fewer arrests and have a greater impact on crime if you are targeting the people that are responsible for committing the crime as opposed to just generating the numbers," he said.
Earlier this year, Ramsey shook up the department's command staff, appointing new captains in some of the city's toughest police districts.
The department has asked the captains to pay special attention to crime in 35 newly identified violence hot spots.
"We've identified them as being the most problematic," said First Deputy Police Commissioner Richard J. Ross Jr. "What we have done is drill down on these spots and are holding the captains accountable for what crime occurs."
To support the commanders, Ramsey has also equipped captains with "embedded" intelligence officers, as well as crime analysts trained at Temple University.
The department has sought to spread accountability by asking midlevel supervisors - lieutenants and sergeants - to take responsibility for crime-fighting plans for specific geographic areas.
The new strategy has paid dramatic dividends in North Philadelphia's 22d Police District. There, commanders used analysis to conclude that crime in the district tended to spike in the spring, prompting them to increase foot patrols.
That shift drove a reduction in homicide in the district to seven from 21 for the same period in 2012.
In recent years, there has been a "cementing of good strategic thinking within the Philadelphia Police Department," said Jerry Ratcliffe, a Temple University criminologist who consults with the department on "smart policing."
Department commanders, he said, have moved away from the "tyranny of the urgent" - chasing the most recent dots on the crime maps - and are asking more "searching, strategic questions about repeat offenders and long-term trouble spots."
Among other reforms, District Attorney Williams began a program known as Gunstat in 2012 aimed at using crime statistics and other intelligence to identify and crack down on the most serious gun criminals. The program imposes harsher penalties on the hundreds convicted every year of carrying an illegal firearm.
"If you're doing a better job on the guys carrying guns, you're going to have an impact on murder," said Brian Lentz, the assistant district attorney who heads the crackdown.
In response to demands from prosecutors, Municipal Court judges have imposed significantly higher bail on people charged with illegal gun possession, quadrupling the average amount levied. Prosecutors have also been using gun arrests as a reason to jail suspects as probation violators and winning longer sentences for new convictions.
"We are getting some dramatic results against these offenders," Lentz said. "The more you are disrupting, the less gun crime you're going to have."
As just one example, he cited the office's recent focus on a North Philadelphia man, Shaheed Springs, 21, who had beaten a string of arrests in recent years, including a case in which he was charged with shooting a man in the throat. That last case collapsed, a prosecutor said, when "our witnesses dropped off the face of the earth."
This year, Gunstat prosecutor Tanner Rouse pursued Spring aggressively, even retrying him on a gun-possession charge after a hung jury in an initial trial. To build another case, Rouse tracked down a tape of a call from jail in which Springs spoke with a girlfriend about his .40-caliber Smith & Wesson handgun.
For a May sentencing on both convictions, Rouse brought in a succession of police officers who had dealt with Springs. Among them was an officer who testified that when he arrested Springs on a 2010 gun charge, Springs said: "It's all good. When I get out, I'm gonna get another gun and shoot police and you better . . . bring backup." The judge sentenced Springs to 7½ to 17 years in prison.
While Williams was launching Gunstat, Chief Justice Ronald D. Castille of the State Supreme Court and Seamus P. McCaffery, the two Philadelphians on the high court, were pushing through wide-ranging reforms in the city criminal courts.
They acted in response to a 2009 Inquirer investigative series that reported that Philadelphia had the nation's lowest conviction rate for violent crime among the nation's largest cities.
The high court pushed through rules changes aimed at ending the legal gamesmanship in which defense attorneys sought to win cases though delay. They set up a special court to crack down on fugitives and brought in firms to collect millions of dollars in unpaid bail. They took steps to attack witness intimidation, including authorizing the use of secret indicting grand juries.
Consultant William G. Chadwick Jr., a former top city prosecutor who has helped guide the systemic overhaul, said word of the changes seemed to be getting out on the street.
"It is highly encouraging to see violent crime rates falling after the massive court reforms that were implemented under the leadership of the Supreme Court," Chadwick said in a statement.
"Although many factors influence crime rates, the reforms have most certainly contributed to the decline. Fewer cases are breaking down; defendants are being held accountable when they fail to appear; new tools are available to protect witnesses from intimidation; and more cases are being adjudicated promptly on their merits."
Bilal Qayyum, president of the Father's Day Rally Committee, a group that works to curb violence, greeted the trend cautiously.
"I'm happy these numbers are down," he said. "But I also understand that all we need is a couple of weeks of heavy gunplay, and those can go up again."
Even so, Qayyum said, the crackdown by police, prosecutors, and judges, as well as such factors as improved trauma care at city hospitals and neighborhood anger at violence, seemed to be converging to save lives.
"I just think it's a combination of all of these efforts at work," he said. "We're beginning to see the impact."
Contact Mike Newall at 215-854-2759 or firstname.lastname@example.org., or follow @MikeNewall on Twitter.